Atomic Structure, Bonding & Chemical Changes

What is found in the nucleus of an atom?

• ANeutrons only, giving the nucleus no charge and almost all the atom's mass
• BProtons and neutrons, giving the nucleus a positive charge and almost all the atom's mass✓
• CElectrons and neutrons, giving the nucleus a negative charge and almost all the atom's mass
• DProtons and electrons, giving the nucleus a neutral charge and almost all the atom's mass, despite containing charged particles

Answer: An atom's nucleus contains protons and neutrons, which together account for almost all of the atom's mass. The protons give the nucleus its overall positive charge, since neutrons carry no charge. Electrons carry a negative charge but occupy the space around the nucleus rather than sitting inside it, and a nucleus is never neutral or charge-free because it always contains at least one proton.

What are the relative charges of a proton, a neutron and an electron?

• AA proton has a relative charge of -1, a neutron has no charge, and an electron has a relative charge of +1, giving the atom an overall negative nuclear charge
• BA proton has no charge, a neutron has a relative charge of +1, and an electron has a relative charge of -1, so the nucleus itself would carry no overall charge
• CA proton has a relative charge of +1, a neutron has no charge, and an electron has a relative charge of -1✓
• DA proton has a relative charge of +1, a neutron has a relative charge of -1, and an electron has no charge, so charge would depend only on the number of protons present

Answer: Protons carry a relative charge of +1, neutrons carry no charge, and electrons carry a relative charge of -1. This combination gives a neutral atom an equal number of protons and electrons so the charges balance overall. Swapping which particle is positive, negative or uncharged does not match the accepted relative charges used throughout the periodic table.

How do the relative masses of protons, neutrons and electrons compare?

• AProtons, neutrons and electrons all have the same relative mass of approximately 1, meaning an atom's mass would depend equally on every particle it contains, electrons included
• BProtons and neutrons have a relative mass of approximately 1, while electrons have a relative mass that is very small by comparison✓
• CNeutrons and electrons have a relative mass of approximately 1, while protons have a relative mass that is very small by comparison, contributing almost nothing to atomic mass
• DProtons and electrons have a relative mass of approximately 1, while neutrons have a relative mass that is very small by comparison, contributing almost nothing to atomic mass

Answer: Protons and neutrons both have a relative mass of about 1, so together they account for almost all of an atom's mass. Electrons have a relative mass that is many thousands of times smaller, so their contribution to atomic mass is negligible. Claiming electrons match protons or neutrons in mass, or that all three particles are equal, misrepresents this large difference in scale.

A sodium atom has an atomic number of 11 and a mass number of 23. How many neutrons does it contain?

• A12✓
• B11
• C34
• D23

Answer: The number of neutrons in an atom equals its mass number minus its atomic number. For this sodium atom, 23 minus 11 gives 12 neutrons. Eleven is instead the number of protons and electrons, twenty-three is the mass number itself, and thirty-four wrongly adds the two numbers together rather than subtracting them.

What determines an element's position in the periodic table?

• AAn element's position in the periodic table is determined by its atomic number, which matches its arrangement of electrons✓
• BAn element's position in the periodic table is determined by its number of neutrons, which matches its arrangement of electrons
• CAn element's position in the periodic table is determined by its mass number, which matches its arrangement of electrons
• DAn element's position in the periodic table is determined by its relative atomic mass alone, regardless of electron arrangement

Answer: Elements in the periodic table are arranged in order of atomic number, which is the number of protons and therefore also the number of electrons in a neutral atom. This ordering places elements with similar electron arrangements, and so similar chemical properties, into the same group. Mass number and neutron count vary between isotopes of the same element, so neither could fix an element's set position.

What is an isotope?

• AAn atom of a different element with the same number of protons and neutrons as another element's atoms
• BAn atom of an element with the same number of protons but a different number of electrons than other atoms of that element
• CAn atom of an element with the same number of neutrons as other atoms of that element but a different number of protons
• DAn atom of an element with the same number of protons as other atoms of that element but a different number of neutrons✓

Answer: Isotopes share the same number of protons, which is what defines an element, but they differ in their number of neutrons. Because their proton and electron numbers stay the same, isotopes have identical chemical properties but slightly different masses. Changing the proton number would actually change the element, and altering the electron number would create an ion rather than an isotope.

Why did the discovery of isotopes require a change to Mendeleev's original way of arranging elements?

• AMendeleev ordered elements by relative atomic mass, and isotopes show that atoms of one element can have different masses✓
• BMendeleev ordered elements by electron arrangement, and isotopes show that electrons are shared between different elements
• CMendeleev ordered elements by boiling point, and isotopes show that this property is fixed for every element
• DMendeleev ordered elements by number of neutrons, and isotopes show that this number never varies within an element

Answer: Mendeleev's original table ordered elements mainly by increasing relative atomic mass. The existence of isotopes, atoms of the same element with different masses, meant that ordering strictly by mass could put elements out of step with their chemical properties, so the modern table instead orders elements by atomic number. Neutron count, electron sharing and boiling point were never the basis of Mendeleev's original ordering.

Why does reactivity increase down Group 1 of the periodic table?

• ABecause each atom gains an extra outer electron that is progressively easier to lose
• BBecause the outer electron is progressively closer to the nucleus and so is easier to lose, contrary to the actual trend observed
• CBecause the outer electron is progressively further from the nucleus and so is easier to lose✓
• DBecause the number of protons decreases, so the nucleus attracts the outer electron less strongly

Answer: Group 1 elements, the alkali metals, become more reactive going down the group. Each element down the group has its outer electron in a shell further from the nucleus, so that electron is held less tightly and is lost more easily during reaction. Group 1 atoms all have one outer electron rather than gaining more, and the proton number increases down the group rather than decreasing.

Why does reactivity decrease down Group 7 of the periodic table?

• ABecause the outer shell is progressively closer to the nucleus, making an extra electron harder to attract as the shells become more compact going down the group
• BBecause the outer shell is progressively further from the nucleus, making an extra electron harder to attract✓
• CBecause each element loses an outer electron more readily going down the group, similar to the pattern seen in Group 1 metals
• DBecause the halogens gain an additional full shell of electrons at every step down the group, increasing their reactivity each time

Answer: Group 7 elements, the halogens, become less reactive going down the group. As the outer shell gets further from the nucleus, the attraction for an incoming electron weakens, so it becomes harder for the atom to gain the extra electron needed to react. Halogens react by gaining, not losing, an electron, and they do not gain a whole extra full shell at every step, only one more occupied shell.

Why are the noble gases in Group 0 so unreactive?

• AThey have a full outer shell of electrons, giving them an unstable arrangement that reacts readily with other elements
• BThey have a partly filled outer shell, giving them a stable arrangement that does not readily gain or lose electrons
• CThey have a full outer shell of electrons, giving them a stable arrangement that does not readily gain or lose electrons✓
• DThey have one electron in their outer shell, giving them a stable arrangement that does not readily gain or lose electrons

Answer: The noble gases in Group 0 have a full outer shell of electrons, which makes them very stable and largely unreactive. This stability is also why they exist as individual atoms rather than bonding together into molecules. They are not metals with one outer electron, their outer shell is not merely partly filled, and a full outer shell gives stability rather than instability.

How do metals and non-metals typically differ in their properties?

• AMetals typically do not conduct electricity, while non-metals typically conduct well and form positive ions, making metals behave chemically like typical non-metals
• BMetals typically conduct electricity and form positive ions, while non-metals typically do not conduct and tend to form negative ions✓
• CMetals and non-metals both conduct electricity equally well and form ions of the same charge as each other, despite their very different positions in the periodic table
• DMetals typically conduct electricity and form negative ions, while non-metals typically do not conduct and tend to form positive ions, contrary to their actual behaviour

Answer: Metals are generally good conductors of electricity and lose electrons to form positive ions, reflecting their position on the left and centre of the periodic table. Non-metals are generally poor conductors, with the exception of graphite, and tend to gain electrons to form negative ions. Reversing which group conducts or forms which charge of ion, or claiming both behave identically, misrepresents this periodic-table-wide pattern.

How are particles arranged and how do they move in a solid compared with a gas?

• AIn solids the particles are held in fixed positions close together, while in gases they move freely and are far apart✓
• BIn solids the particles move freely and are far apart, while in gases they are held in fixed positions close together
• CIn solids the particles are arranged in fixed positions close together, and in gases they are also arranged in fixed positions
• DIn solids the particles move freely and are close together, while in gases they are held in fixed positions far apart

Answer: The particle model describes solids as having particles held in fixed positions, close together and vibrating on the spot, which gives them a fixed shape. Gas particles, by contrast, move freely at high speed and are spread far apart, which is why gases have no fixed shape or volume. Solid particles are never described as moving freely, and gas particles are never described as fixed in position.

How does the strength of the forces between particles affect a substance's melting and boiling points?

• ASubstances with stronger forces between their particles generally need less energy to melt or boil, giving lower melting and boiling points, contrary to the accepted pattern
• BSubstances with weaker forces between their particles generally need more energy to melt or boil, giving higher melting and boiling points, though this contradicts the general trend
• CThe strength of the forces between particles has no effect at all on melting or boiling point, even though experimental data clearly show otherwise
• DSubstances with stronger forces between their particles generally need more energy to melt or boil, giving higher melting and boiling points✓

Answer: Melting and boiling involve overcoming the forces holding particles together, so substances with stronger interparticle forces require more energy to separate their particles, giving higher melting and boiling points. Weaker forces need less energy to overcome and so produce lower melting and boiling points. It is not true that stronger forces need less energy, or that force strength makes no difference, since this pattern explains why giant structures have such high melting points compared with simple molecules.

Why is melting ice classed as a physical change rather than a chemical change?

• ABecause it releases a gas, which is always a sign that a chemical change has taken place
• BBecause it produces a new substance that cannot be reversed by freezing the water again
• CBecause it changes the chemical formula of the water into a different compound
• DBecause it does not produce a new substance and can easily be reversed by freezing the water again✓

Answer: A physical change, such as melting ice into water, rearranges or moves particles without forming a new substance, and it can usually be reversed, as freezing shows. A chemical change, such as burning or rusting, produces one or more new substances with different properties that are often hard to reverse. Melting ice releases no gas and does not alter water's chemical formula, so neither of those descriptions applies here.

What happens to electrons when an ionic bond forms between a metal and a non-metal atom?

• AElectrons are shared equally between the metal atom and the non-metal atom, creating oppositely charged ions
• BElectrons become delocalised and move freely between many metal atoms arranged in a regular structure
• CElectrons are transferred from the metal atom to the non-metal atom, creating oppositely charged ions✓
• DElectrons are transferred from the metal atom to the non-metal atom, creating two ions of the same charge

Answer: In ionic bonding, electrons are transferred from a metal atom to a non-metal atom, producing a positive ion and a negative ion that attract each other strongly. Sharing electrons equally instead describes covalent bonding, and the transfer always produces ions of opposite, not the same, charge. A sea of delocalised electrons moving between many metal atoms describes metallic bonding rather than an ionic bond.

What holds two atoms together in a covalent bond?

• AA pair of protons shared between the nuclei of the two bonded atoms, an arrangement that never occurs in real atoms
• BA pair of electrons transferred completely from one atom to the other, as happens instead in ionic compounds
• CA sea of delocalised electrons surrounding both of the bonded atoms, as found instead in metallic structures
• DA shared pair of electrons that is attracted to the nucleus of each bonded atom✓

Answer: A covalent bond forms when two atoms, usually of non-metals, share a pair of electrons, with the shared electrons attracted to both nuclei. Complete transfer of electrons describes ionic bonding instead, and protons remain fixed within the nucleus so they cannot be shared between atoms. A sea of delocalised electrons describes metallic bonding, not a covalent bond between two atoms.

How is a metal structure held together, and why does this allow metals to conduct electricity?

• APositive metal ions are held together only by fixed shared pairs of electrons between neighbouring ions
• BNeutral metal atoms are arranged randomly with no free-moving particles able to carry charge
• CPositive metal ions are arranged in a regular structure surrounded by a sea of delocalised electrons that can carry charge✓
• DNegative metal ions are arranged in a regular structure surrounded by a sea of delocalised protons that can carry charge

Answer: In a metal, the outer electrons become delocalised and are free to move throughout the whole structure, leaving behind a regular arrangement of positive metal ions. The strong attraction between these positive ions and the sea of delocalised electrons explains why metals conduct electricity and can be bent or shaped. Protons cannot delocalise or move between ions, and a random arrangement with no free particles would not conduct at all.

Why do giant covalent structures such as diamond have very high melting points?

• AOnly weak intermolecular forces exist between separate small molecules in the structure, so little energy would be needed to separate them
• BStrong metallic bonds spread through a sea of delocalised electrons throughout the structure, as in a typical metal
• CA single weak covalent bond holds the entire giant structure together at one point, with no other bonds present anywhere else
• DA huge number of strong covalent bonds extend throughout the structure and must be broken to melt it✓

Answer: Substances such as diamond and silicon dioxide form giant covalent structures in which every atom is linked to neighbouring atoms by strong covalent bonds extending throughout the whole structure. Melting requires breaking a vast number of these strong bonds, which needs a huge amount of energy and gives extremely high melting points. Giant covalent structures do not contain metallic bonds or rely on weak forces between separate molecules, and they are held together by many bonds rather than just one.

Why can carbon form such a huge range of different chain and ring compounds?

• AEach carbon atom forms four ionic bonds, allowing carbon atoms to link together in many different arrangements
• BEach carbon atom forms four covalent bonds but can only ever form straight chains, never rings
• CEach carbon atom forms two covalent bonds, allowing carbon atoms to link together in many different arrangements
• DEach carbon atom forms four covalent bonds, allowing carbon atoms to link together in many different arrangements✓

Answer: Each carbon atom can form four covalent bonds, which lets carbon atoms link to one another and to other elements in an enormous variety of chains and rings. This bonding versatility is the basis of the whole of organic chemistry, from simple hydrocarbons to complex polymers. Carbon does not form only two bonds, it does not bond ionically in organic compounds, and it is well known to form ring structures as well as chains.

Why is diamond extremely hard but unable to conduct electricity?

• AEvery carbon atom forms four strong covalent bonds in a rigid lattice, leaving delocalised electrons free to carry charge
• BDiamond is made of small individual carbon molecules held together by weak forces, similar to the structure of simple molecular substances like ice
• CEvery carbon atom forms four strong covalent bonds in a rigid lattice, leaving no free electrons to carry charge✓
• DDiamond is made of weakly bonded layers of carbon atoms that can slide over each other

Answer: In diamond, each carbon atom bonds covalently to four others in a rigid three-dimensional lattice, making it extremely hard. Because all outer electrons are held firmly within these fixed bonds, there are no free electrons or ions to carry a charge, so diamond does not conduct electricity. Diamond is not made of sliding layers, that description fits graphite, and it is a single giant structure rather than a collection of small molecules.

Why does graphite conduct electricity and feel slippery, unlike diamond?

• AIts carbon atoms form weakly bonded layers, and each atom has one delocalised electron free to move within a layer✓
• BIts carbon atoms form strongly bonded layers, and each atom has one delocalised electron free to move within a layer
• CIts carbon atoms each form four covalent bonds within a rigid lattice, leaving no delocalised electrons
• DIts carbon atoms are arranged as long tangled polymer chains rather than as flat layers

Answer: In graphite, each carbon atom forms only three covalent bonds within a layer, leaving one delocalised electron per atom that is free to move and carry a charge, which is why graphite conducts electricity. The layers themselves are only weakly held together, so they can slide over each other, giving graphite its slippery feel. Its layers are weakly, not strongly, bonded to each other, and graphite is a layered carbon structure rather than a polymer.

What structural feature of graphene gives it very high strength and excellent electrical conductivity?

• AIt is a hollow, ball-shaped cage of carbon atoms designed to trap and deliver drug molecules
• BIt is a three-dimensional lattice of carbon atoms identical in structure to diamond, making it just as hard and just as unable to conduct electricity
• CIt is a single layer, one atom thick, of carbon atoms arranged in a hexagonal pattern with delocalised electrons✓
• DIt is a single layer, one atom thick, of carbon atoms arranged in a hexagonal pattern with no delocalised electrons

Answer: Graphene consists of a single layer of carbon atoms, one atom thick, arranged in a hexagonal pattern, which makes it extremely strong for its mass and an excellent conductor of electricity because of its delocalised electrons. A hollow cage structure that can carry drug molecules instead describes a fullerene such as buckminsterfullerene. Graphene's flat single-layer structure is quite different from the three-dimensional lattice of diamond, and it does have delocalised electrons available to carry charge.

What does conservation of mass mean for a balanced chemical equation?

• AMass only balances in reactions that do not produce a gas as one of their products, regardless of whether the system is open or closed
• BThe total mass of the reactants is always greater than the total mass of the products, because atoms are lost during the reaction
• CThe total mass of the reactants equals the total mass of the products, because atoms are never created or destroyed✓
• DThe total mass of the reactants is always less than the total mass of the products, because new atoms form during the reaction

Answer: Balancing a chemical equation reflects the fact that atoms are never created or destroyed in a reaction, only rearranged, so the total mass of reactants must equal the total mass of products. This conservation of mass holds true whether or not a gas is produced, provided the reaction takes place in a closed system. Reactant mass is never greater than or less than product mass in a correctly balanced equation, since no atoms are gained or lost overall.

A glowing splint relights when placed into a test tube of gas. Which gas is present?

• AThe gas produced is hydrogen
• BThe gas produced is oxygen✓
• CThe gas produced is carbon dioxide
• DThe gas produced is chlorine

Answer: Oxygen relights a glowing splint, because the extra oxygen allows the splint to burst back into flame. Carbon dioxide instead turns limewater cloudy and would extinguish rather than relight a glowing splint, and hydrogen is tested for with a lit splint producing a squeaky pop, not a glowing splint relighting. Chlorine is identified separately because it bleaches damp litmus paper.

A lit splint held at the mouth of a test tube of gas produces a squeaky pop. Which gas is present?

• AThe gas produced is carbon dioxide
• BThe gas produced is chlorine
• CThe gas produced is hydrogen✓
• DThe gas produced is oxygen

Answer: Hydrogen gas gives a characteristic squeaky pop when a lit splint is held at the mouth of the test tube, caused by the rapid combustion of the hydrogen. Chlorine instead bleaches damp litmus paper white and does not produce this pop test, while a glowing splint relighting tests for oxygen rather than hydrogen. Carbon dioxide is identified separately because it turns limewater cloudy.

Limewater turns cloudy when a gas is bubbled through it. Which gas is being tested for?

• AThe gas produced is oxygen
• BThe gas produced is chlorine
• CThe gas produced is hydrogen
• DThe gas produced is carbon dioxide✓

Answer: Bubbling a gas through limewater and observing it turn cloudy is the standard test for carbon dioxide, caused by the formation of insoluble calcium carbonate. Oxygen is instead tested for using a glowing splint that relights, and hydrogen is tested for using a lit splint that produces a squeaky pop. Chlorine is identified with damp litmus paper, which it bleaches white rather than clouding limewater.

Damp litmus paper is bleached and turns white when placed into a test tube of gas. Which gas is present?

• AThe gas produced is chlorine✓
• BThe gas produced is carbon dioxide
• CThe gas produced is hydrogen
• DThe gas produced is oxygen

Answer: Chlorine gas bleaches damp litmus paper, turning it white, because of its strong bleaching action. Oxygen instead relights a glowing splint and does not bleach litmus paper at all, while hydrogen produces a squeaky pop with a lit splint rather than affecting litmus. Carbon dioxide is identified separately because it turns limewater cloudy, not because it bleaches litmus.

What are the products when a reactive metal reacts with a dilute acid?

• AA salt and carbon dioxide gas
• BA salt and oxygen gas
• COnly water, with no gas released
• DA salt and hydrogen gas✓

Answer: A reactive metal reacting with a dilute acid forms a salt and hydrogen gas, which can be confirmed with the squeaky-pop test. Oxygen and carbon dioxide are not the gases released in this reaction, since those form in different processes such as combustion or carbonate reactions. A metal-acid reaction always releases a gas rather than producing only water with nothing else.

What are the products when an acid reacts with a metal carbonate?

• AOnly a salt, with no other products formed
• BA salt, water and hydrogen gas
• CA salt and oxygen gas, with no water formed
• DA salt, water and carbon dioxide gas✓

Answer: Acids react with metal carbonates to form a salt, water and carbon dioxide gas, which can be confirmed because the gas turns limewater cloudy. Hydrogen gas is instead released when acids react with reactive metals, not with carbonates, and oxygen gas is not a product of this reaction at all. The reaction always produces three products together, never just a salt on its own.

What happens to hydrogen ions and hydroxide ions during neutralisation?

• AHydrogen ions from one acid combine with hydrogen ions from a second acid to form water
• BHydrogen ions from the acid combine with hydroxide ions from the alkali to form water✓
• CHydrogen ions from the acid combine with hydroxide ions from the alkali to form oxygen gas
• DHydroxide ions from the alkali combine with metal ions from the acid to form water

Answer: Neutralisation happens when an acid reacts with an alkali, and the hydrogen ions from the acid combine with hydroxide ions from the alkali to form water, leaving a salt in solution. This ion combination produces water, not oxygen gas, and neutralisation specifically requires an acid reacting with an alkali rather than two acids together. Metal ions are not part of this particular ion combination, since it is the hydrogen and hydroxide ions that join to form water.

How can displacement reactions be used to work out the reactivity series of metals experimentally?

• AMetals cannot displace each other from salt solutions, so melting point must be used instead
• BA less reactive metal displaces a more reactive metal from a solution of the more reactive metal's salt
• CAny two metals displace each other equally from a solution of either metal's salt
• DA more reactive metal displaces a less reactive metal from a solution of the less reactive metal's salt✓

Answer: In a displacement reaction, a more reactive metal takes the place of a less reactive metal in a compound, such as when iron displaces copper from copper sulfate solution, and this behaviour can be used to build up a reactivity series experimentally. A less reactive metal cannot displace a more reactive one, and displacement reactions are not equal in both directions between any two metals. Melting point is not the property used to determine reactivity order.

What happens to the ions during the electrolysis of a molten ionic compound?

• AThe ions stay fixed in place, so the compound cannot be split by passing a current through it
• BPositive ions move to the cathode and negative ions move to the anode, splitting the compound into its elements✓
• CBoth positive and negative ions move to the cathode, splitting the compound into its elements
• DPositive ions move to the anode and negative ions move to the cathode, splitting the compound into its elements

Answer: When an ionic compound is molten, its ions are free to move, so passing a current through it causes positive ions to migrate to the negative cathode and negative ions to migrate to the positive anode, where they are discharged as elements. Positive and negative ions are attracted to opposite electrodes, not the same one, so they do not both travel to the cathode. Melting frees the ions from the fixed lattice, which is exactly why electrolysis becomes possible.

During the electrolysis of an aqueous solution, when is hydrogen produced at the cathode instead of the metal?

• AWhenever the solution contains no water at all
• BWhenever the metal in solution is less reactive than hydrogen
• CWhenever the metal in solution is more reactive than hydrogen✓
• DWhenever the electrolysis current is switched off during the process

Answer: During the electrolysis of an aqueous solution, water is also present and can compete with the dissolved metal ions at the cathode. Hydrogen is produced instead of the metal whenever that metal is more reactive than hydrogen, because hydrogen ions are discharged in preference to the more reactive metal ions. Water must be present for this competition to occur at all, and no discharge happens while the current is switched off.

In terms of electron transfer, what happens during oxidation and reduction?

• AOxidation is the gain of electrons by a substance, and reduction is the loss of electrons
• BBoth oxidation and reduction involve the gain of electrons by a substance
• CBoth oxidation and reduction involve the loss of electrons from a substance
• DOxidation is the loss of electrons from a substance, and reduction is the gain of electrons✓

Answer: Redox reactions can be described in terms of electron transfer: oxidation is the loss of electrons from a substance, while reduction is the gain of electrons, a pattern often remembered as OIL RIG. This is consistent with the older definition in terms of oxygen, since a substance that loses electrons often gains oxygen at the same time. Oxidation and reduction are opposite processes, so they cannot both involve losing electrons or both involve gaining electrons in the same reaction.

Iron has atomic number 26. An ion of iron-56 has a 3+ charge (⁵⁶Fe³⁺).

How many protons, neutrons and electrons does this ion have?

• A26 protons, 30 neutrons, 29 electrons
• B26 protons, 56 neutrons, 23 electrons
• C23 protons, 30 neutrons, 26 electrons
• D26 protons, 30 neutrons, 23 electrons✓

Answer: The atomic number gives 26 protons. Neutrons = mass number − atomic number = 56 − 26 = 30. A neutral atom has 26 electrons; a 3+ ion has lost three, leaving 23. Adding three electrons gives 29, which would make a negative ion; 56 is the total number of protons and neutrons, and the number of protons never changes when an ion forms.

Ammonium nitrate, NH₄NO₃, is used as a fertiliser. Its relative formula mass is 80. (Relative atomic masses: H = 1, N = 14, O = 16)

What is the percentage by mass of nitrogen in ammonium nitrate?

• A17.5%
• B35%✓
• C60%
• D47%

Answer: There are two nitrogen atoms in the formula, one in NH₄ and one in NO₃, so the mass of nitrogen = 2 × 14 = 28. Percentage = 28 ÷ 80 × 100 = 35%. The tempting 17.5% counts only one nitrogen atom, 60% is the percentage of oxygen (48 ÷ 80), and 47% comes from 28 ÷ 60, dividing by the wrong total.

Magnesium burns in oxygen: 2Mg + O₂ → 2MgO (Relative atomic masses: Mg = 24, O = 16)

What mass of magnesium oxide is made when 6.0 g of magnesium burns completely?

• A20 g
• B5.0 g
• C4.0 g
• D10 g✓

Answer: Moles of Mg = 6.0 ÷ 24 = 0.25 mol. The equation shows 2 mol Mg makes 2 mol MgO, a 1 : 1 ratio, so 0.25 mol MgO forms. Mass = 0.25 × (24 + 16) = 10 g. The tempting 20 g doubles the moles because of the 2 in front of MgO, though there is also a 2 in front of Mg; 5.0 g halves them, and 4.0 g is the mass of oxygen that combines with the magnesium.

4.8 g of magnesium is added to a solution containing 0.10 mol of hydrochloric acid. Mg + 2HCl → MgCl₂ + H₂ (Relative atomic mass: Mg = 24)

Which reactant is limiting, and how many moles of hydrogen are produced?

• AMagnesium is limiting; 0.20 mol of hydrogen
• BHydrochloric acid is limiting; 0.10 mol of hydrogen
• CHydrochloric acid is limiting; 0.050 mol of hydrogen✓
• DMagnesium is limiting; 0.050 mol of hydrogen

Answer: Moles of Mg = 4.8 ÷ 24 = 0.20 mol. Reacting all of it would need 0.40 mol of HCl, but only 0.10 mol is present, so the acid runs out first and is limiting. 0.10 mol HCl reacts with 0.050 mol Mg and, from the 2 : 1 ratio, makes 0.050 mol of H₂; the rest of the magnesium is left over. The tempting 0.10 mol ignores the 2 in front of HCl.

A student dissolves 8.0 g of sodium hydroxide in water and makes the solution up to 250 cm³.

What is the concentration of the solution?

• A2.0 g/dm³
• B0.032 g/dm³
• C32 g/dm³✓
• D8.0 g/dm³

Answer: Convert the volume to dm³: 250 cm³ = 0.250 dm³. Concentration = mass ÷ volume = 8.0 g ÷ 0.250 dm³ = 32 g/dm³. The tempting 2.0 multiplies the mass by the volume in dm³, 0.032 is the concentration in g/cm³ (8.0 ÷ 250), and 8.0 g/dm³ treats 250 cm³ as if it were 1 dm³.

Hydrochloric acid neutralises potassium hydroxide solution.

Which is the ionic equation for this reaction?

• AK⁺(aq) + Cl⁻(aq) → KCl(aq)
• BH⁺(aq) + OH⁻(aq) → H₂O(l)✓
• CHCl(aq) + OH⁻(aq) → Cl⁻(aq) + H₂O(l)
• DH⁺(aq) + KOH(aq) → K⁺(aq) + H₂O(l)

Answer: In solution, the acid is H⁺ and Cl⁻ ions and the alkali is K⁺ and OH⁻ ions. The K⁺ and Cl⁻ ions are spectator ions: they stay dissolved and unchanged, so they are left out. The only change is hydrogen ions reacting with hydroxide ions to form water, the ionic equation for every acid–alkali neutralisation. Writing HCl or KOH as molecules ignores that both are fully ionised in solution.