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QCE Chemistry practice questions

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Question 1Chemical fundamentals: structure, properties and reactions

Which particle is transferred when a sodium atom forms a sodium ion in a reaction with chlorine?

  1. One neutron
  2. One electron pair
  3. One proton
  4. One electron Correct answer.

Explanation

Metal atoms such as sodium lose valence electrons to achieve a stable noble-gas electron configuration, forming a positively charged cation. A single electron is transferred from sodium to chlorine, producing Na+ and Cl-. Protons and neutrons reside in the nucleus and are not transferred in ordinary chemical reactions.

Question 2Chemical fundamentals: structure, properties and reactions

An atom of an element has the electron configuration 2,8,7. In which group of the periodic table does this element belong?

  1. Group 7
  2. Group 15
  3. Group 1
  4. Group 17 Correct answer.

Explanation

The number of valence electrons in the outermost shell determines the group for main-group elements. With seven valence electrons, the element belongs to Group 17, the halogens. Group numbers in the modern IUPAC system count valence electrons across the p-block.

Question 3Chemical fundamentals: structure, properties and reactions

Which type of bonding is present in solid magnesium oxide?

  1. Covalent bonding
  2. Hydrogen bonding
  3. Ionic bonding Correct answer.
  4. Metallic bonding

Explanation

Magnesium oxide is formed from a metal (magnesium) and a non-metal (oxygen). Electrons are transferred from magnesium to oxygen, producing Mg2+ and O2- ions held together in a giant ionic lattice by strong electrostatic attraction.

Question 4Chemical fundamentals: structure, properties and reactions

Which statement best describes the structure of a diamond?

  1. Layers of carbon atoms held by weak forces
  2. A lattice of carbon ions in a sea of electrons
  3. Discrete C2 molecules held by covalent bonds
  4. A giant covalent lattice of carbon atoms Correct answer.

Explanation

Diamond consists of carbon atoms each covalently bonded to four others in a rigid three-dimensional network. This giant covalent lattice gives diamond its extreme hardness and very high melting point. There are no discrete molecules or free ions in diamond.

Question 5Chemical fundamentals: structure, properties and reactions

Which intermolecular force is primarily responsible for the relatively high boiling point of water compared with hydrogen sulfide?

  1. London dispersion forces only
  2. Ion-dipole forces
  3. Hydrogen bonding Correct answer.
  4. Metallic bonding

Explanation

Water molecules form hydrogen bonds because hydrogen is bonded to a highly electronegative oxygen atom with lone pairs. These strong intermolecular attractions require more energy to overcome, raising the boiling point relative to hydrogen sulfide, which lacks hydrogen bonding.

Question 6Chemical fundamentals: structure, properties and reactions

In the reaction 2H2 + O2 -> 2H2O, how many moles of water are produced when 3.0 mol of hydrogen reacts completely with excess oxygen?

  1. 6.0 mol
  2. 2.0 mol
  3. 1.5 mol
  4. 3.0 mol Correct answer.

Explanation

The balanced equation shows that 2 mol of hydrogen produces 2 mol of water, giving a 1:1 mole ratio. Therefore 3.0 mol of hydrogen produces 3.0 mol of water when oxygen is in excess.

Question 7Chemical fundamentals: structure, properties and reactions

Which observation indicates that a chemical reaction has occurred when dilute hydrochloric acid is added to magnesium ribbon?

  1. The temperature of the acid decreases
  2. The acid becomes more concentrated
  3. The magnesium becomes wet
  4. Bubbles of gas are produced and the metal dissolves Correct answer.

Explanation

Reaction of magnesium with hydrochloric acid produces hydrogen gas and magnesium chloride, so effervescence (bubbles) and dissolution of the metal are evidence of a chemical change. A new substance, hydrogen, is formed. Physical wetting or dilution alone does not indicate reaction.

Question 8Chemical fundamentals: structure, properties and reactions

Which property is characteristic of a strong acid in aqueous solution?

  1. It ionises almost completely to release hydrogen ions Correct answer.
  2. It must contain carbon atoms
  3. It has a very high pH value
  4. It reacts only with metals

Explanation

A strong acid ionises essentially completely in water, producing a high concentration of hydrogen ions (hydronium ions). This near-complete ionisation is what distinguishes strong acids from weak acids, which ionise only partially.

Question 9Chemical fundamentals: structure, properties and reactions

Which of the following is a correct Lewis electron-dot structure for the ammonia molecule, NH3?

  1. N bonded to three H atoms with no lone pair on N
  2. N bonded to four H atoms with no lone pair on N
  3. N bonded to three H atoms with one lone pair on N Correct answer.
  4. N bonded to two H atoms with two lone pairs on N

Explanation

Nitrogen contributes five valence electrons and each hydrogen contributes one, giving eight electrons total. Three bonding pairs link N to H, and the remaining two electrons form one lone pair on nitrogen, satisfying the octet rule.

Question 10Chemical fundamentals: structure, properties and reactions

Which molecule has a trigonal planar shape?

  1. BF3 Correct answer.
  2. H2O
  3. CH4
  4. NH3

Explanation

Boron trifluoride has three bonding pairs and no lone pairs on the central boron atom, so electron-pair repulsion gives a trigonal planar arrangement with bond angles of 120 degrees. The other molecules have lone pairs or four bonds that produce different geometries.

Question 11Chemical fundamentals: structure, properties and reactions

Why does the boiling point of the noble gases increase from helium to radon?

  1. The atoms form hydrogen bonds
  2. The atoms become more polar
  3. The number of electrons increases, strengthening dispersion forces Correct answer.
  4. The ionisation energy increases

Explanation

Noble gas atoms are non-polar, so the only intermolecular attraction is London dispersion forces. Larger atoms have more electrons and a more polarisable electron cloud, which strengthens dispersion forces and raises boiling points down the group.

Question 12Chemical fundamentals: structure, properties and reactions

What is the oxidation state of chromium in the dichromate ion, Cr2O7^2-?

  1. +6 Correct answer.
  2. +7
  3. +3
  4. +12

Explanation

Each oxygen is assigned -2, giving -14 for seven oxygens. The overall charge is -2, so the two chromium atoms together must contribute +12, meaning each chromium has an oxidation state of +6.

Question 13Chemical fundamentals: structure, properties and reactions

In the reaction Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s), which species is oxidised?

  1. Zn2+
  2. Zn Correct answer.
  3. Cu
  4. Cu2+

Explanation

Zinc loses two electrons, increasing its oxidation state from 0 to +2, so it is oxidised. Copper ions gain those electrons and are reduced from +2 to 0. Oxidation corresponds to loss of electrons.

Question 14Chemical fundamentals: structure, properties and reactions

A reaction has an enthalpy change of -450 kJ per mole. Which statement about this reaction is correct?

  1. It is endothermic and releases heat
  2. It is endothermic and absorbs heat
  3. It is exothermic and absorbs heat
  4. It is exothermic and releases heat Correct answer.

Explanation

A negative enthalpy change means the system loses energy to the surroundings, which is the defining feature of an exothermic reaction. The magnitude, 450 kJ per mole, indicates the quantity of heat released.

Question 15Chemical fundamentals: structure, properties and reactions

Which factor increases the rate of a chemical reaction between a solid and a solution?

  1. Decreasing the temperature of the solution
  2. Removing the catalyst
  3. Increasing the surface area of the solid Correct answer.
  4. Increasing the particle size of the solid

Explanation

Increasing surface area exposes more particles to collisions, raising the frequency of successful collisions and therefore the reaction rate. Smaller particles or powders provide greater surface area than large lumps.

Question 16Chemical fundamentals: structure, properties and reactions

According to Le Chatelier's principle, what happens when the pressure is increased for the equilibrium N2(g) + 3H2(g) <-> 2NH3(g)?

  1. The reaction stops completely
  2. The equilibrium shifts towards the reactants
  3. The equilibrium shifts towards the products Correct answer.
  4. The equilibrium position is unchanged

Explanation

Increasing pressure favours the direction that produces fewer gas molecules. The left side has four moles of gas and the right side has two, so the equilibrium shifts towards ammonia, the products.

Question 17Chemical fundamentals: structure, properties and reactions

Which pair of species is a conjugate acid-base pair?

  1. H2O and NaCl
  2. HCl and NaOH
  3. NH4+ and NH3 Correct answer.
  4. H2SO4 and SO4^2-

Explanation

A conjugate acid-base pair differs by a single proton. NH4+ donates a proton to become NH3, so these two species form a conjugate pair. The other pairs differ by more than one proton or are not related by proton transfer.

Question 18Chemical fundamentals: structure, properties and reactions

Which statement correctly describes the bonding in a metal such as copper?

  1. Electrons are shared between pairs of metal atoms
  2. Metal ions are held by hydrogen bonds
  3. Positive metal ions are fixed in a lattice surrounded by delocalised electrons Correct answer.
  4. Metal atoms transfer electrons to non-metal atoms

Explanation

In the metallic bonding model, metal cations occupy lattice positions while valence electrons are delocalised and free to move throughout the structure. This sea of electrons explains electrical conductivity and malleability.

Question 19Chemical fundamentals: structure, properties and reactions

A 25.0 mL sample of hydrochloric acid is neutralised by 20.0 mL of 0.100 mol/L sodium hydroxide solution. What is the concentration of the hydrochloric acid?

  1. 0.040 mol/L
  2. 0.080 mol/L Correct answer.
  3. 0.125 mol/L
  4. 0.100 mol/L

Explanation

The reaction is HCl + NaOH -> NaCl + H2O, a 1:1 mole ratio. Moles of NaOH = 0.0200 L x 0.100 mol/L = 0.00200 mol, so moles of HCl = 0.00200 mol. Dividing by 0.0250 L gives 0.080 mol/L.

Question 20Chemical fundamentals: structure, properties and reactions

Which sample contains the greatest number of atoms?

  1. 1.0 mol of helium gas
  2. 1.0 mol of oxygen gas
  3. 1.0 mol of argon gas
  4. 1.0 mol of water Correct answer.

Explanation

One mole of any substance contains Avogadro's number of particles, but the number of atoms per particle differs. Each water molecule contains three atoms, so 1.0 mol of water contains 3.0 mol of atoms, more than the others.

Question 21Chemical fundamentals: structure, properties and reactions

Which explanation best accounts for the very high melting point of sodium chloride?

  1. Covalent bonds between sodium and chlorine atoms
  2. Weak dispersion forces between molecules
  3. Strong electrostatic attractions between oppositely charged ions in a lattice Correct answer.
  4. Hydrogen bonds between chloride ions

Explanation

Sodium chloride is a giant ionic lattice in which each ion is surrounded by ions of opposite charge. Melting requires breaking many strong electrostatic attractions, so a large amount of energy is needed.

Question 22Chemical fundamentals: structure, properties and reactions

Two solutions of equal concentration are tested: solution X has pH 2 and solution Y has pH 4. Which statement about the hydrogen ion concentrations is correct?

  1. X has 2 times the hydrogen ion concentration of Y
  2. X and Y have equal hydrogen ion concentrations
  3. X has 100 times the hydrogen ion concentration of Y Correct answer.
  4. X has 4 times the hydrogen ion concentration of Y

Explanation

pH is a logarithmic scale, so a difference of two pH units corresponds to a hundredfold change in hydrogen ion concentration. Since X has the lower pH, it is more acidic and has 10^2 times the concentration of Y.

Question 23Chemical fundamentals: structure, properties and reactions

A sample of an unknown hydrocarbon is completely combusted, producing 0.20 mol of carbon dioxide and 0.30 mol of water. What is the empirical formula of the hydrocarbon?

  1. CH2
  2. C2H6
  3. C2H3
  4. CH3 Correct answer.

Explanation

All carbon appears as CO2, giving 0.20 mol of carbon atoms, and all hydrogen appears as H2O, giving 2 x 0.30 = 0.60 mol of hydrogen atoms. The ratio C:H simplifies from 0.20:0.60 to 1:3, giving the empirical formula CH3.

Question 24Chemical fundamentals: structure, properties and reactions

Which sequence correctly ranks the bond angles in CH4, NH3 and H2O?

  1. NH3 > CH4 > H2O
  2. CH4 > NH3 > H2O Correct answer.
  3. H2O > NH3 > CH4
  4. CH4 > H2O > NH3

Explanation

Methane has four bonding pairs and an ideal tetrahedral angle of 109.5 degrees. In ammonia and water, lone pairs repel bonding pairs more strongly, compressing the bond angles progressively, so the order is CH4 > NH3 > H2O.

Question 25Chemical fundamentals: structure, properties and reactions

In an electrochemical cell, which statement correctly describes the process occurring at the anode during discharge of a galvanic cell?

  1. Hydrogen ions are always produced
  2. Oxidation occurs as electrons are released Correct answer.
  3. No electron transfer occurs at the anode
  4. Reduction occurs as electrons are gained

Explanation

In any electrochemical cell, oxidation always occurs at the anode, where the electrode loses electrons to the external circuit. In a galvanic cell these released electrons flow through the wire to the cathode, where reduction occurs.

Question 26Chemical fundamentals: structure, properties and reactions

Which subatomic particle determines the identity of an element?

  1. The number of neutrons in the nucleus
  2. The number of electrons in the outer shell
  3. The number of protons in the nucleus Correct answer.
  4. The total number of nucleons in the nucleus

Explanation

The atomic number, equal to the number of protons, uniquely identifies an element and fixes its position in the periodic table. Changing the proton count transforms the atom into a different element, whereas changing neutrons or electrons does not.

Question 27Chemical fundamentals: structure, properties and reactions

An atom of an element has the electron configuration 1s2 2s2 2p6 3s1. In which group and period of the periodic table does this element lie?

  1. Group 2, period 3
  2. Group 1, period 2
  3. Group 17, period 3
  4. Group 1, period 3 Correct answer.

Explanation

The highest occupied shell is the third shell, so the element is in period 3. There is a single electron in the 3s subshell, which places it in group 1, the alkali metals.

Question 28Chemical fundamentals: structure, properties and reactions

Which type of bonding is present in solid potassium chloride?

  1. Metallic bonding between potassium cations and delocalised electrons
  2. Covalent bonding between potassium and chlorine atoms
  3. Ionic bonding between potassium cations and chloride anions Correct answer.
  4. Hydrogen bonding between potassium and chlorine ions

Explanation

Potassium is a metal that transfers an electron to the non-metal chlorine, producing K+ and Cl- ions held together by electrostatic attraction in a lattice. This transfer-and-attraction arrangement is the defining feature of ionic bonding.

Question 29Chemical fundamentals: structure, properties and reactions

A compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass. What is its empirical formula? (Molar masses: C = 12.0, H = 1.0, O = 16.0 g/mol)

  1. CH2O Correct answer.
  2. C2H6O
  3. C2H4O
  4. CHO

Explanation

Dividing each percentage by the molar mass gives mole ratios of 3.33 C, 6.7 H and 3.33 O. Dividing by the smallest value gives 1:2:1, so the empirical formula is CH2O.

Question 30Chemical fundamentals: structure, properties and reactions

Which statement correctly describes the trend in atomic radius across period 3 from sodium to chlorine?

  1. The radius decreases because effective nuclear charge increases while electrons enter the same shell Correct answer.
  2. The radius stays constant because the number of shells does not change
  3. The radius decreases because the number of electron shells decreases
  4. The radius increases because more electrons are added to the atoms

Explanation

Across a period, protons are added to the nucleus while electrons enter the same valence shell, so the effective nuclear charge rises. This stronger attraction pulls the electron cloud inward, reducing the atomic radius.

Question 31Chemical fundamentals: structure, properties and reactions

Which molecule contains a polar covalent bond but has an overall dipole moment of zero?

  1. Water, H2O
  2. Carbon dioxide, CO2 Correct answer.
  3. Hydrogen chloride, HCl
  4. Ammonia, NH3

Explanation

Each carbon-oxygen bond in CO2 is polar, but the linear geometry makes the two bond dipoles equal and opposite, so they cancel. The molecule is therefore non-polar overall despite having polar bonds.

Question 32Chemical fundamentals: structure, properties and reactions

Which intermolecular force is primarily responsible for the relatively high boiling point of ethanol compared with dimethyl ether, which has the same molecular formula?

  1. Hydrogen bonding between ethanol molecules Correct answer.
  2. Ion-dipole attractions between ethanol molecules
  3. Covalent bonding between ethanol molecules
  4. London dispersion forces between ethanol molecules

Explanation

Ethanol has an O-H group, allowing hydrogen bonds to form between molecules, whereas dimethyl ether lacks an O-H bond and cannot hydrogen bond. Stronger intermolecular attractions require more energy to overcome, raising the boiling point.

Question 33Chemical fundamentals: structure, properties and reactions

Which equation correctly represents the first ionisation energy of magnesium?

  1. Mg(s) -> Mg+(g) + e-
  2. Mg(g) -> Mg+(g) + e- Correct answer.
  3. Mg+(g) -> Mg2+(g) + e-
  4. Mg(g) -> Mg2+(g) + 2e-

Explanation

First ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms. The equation must therefore start from gaseous Mg atoms and produce a singly charged gaseous cation plus one electron.

Question 34Chemical fundamentals: structure, properties and reactions

What is the shape of the PF5 molecule, in which phosphorus is bonded to five fluorine atoms?

  1. Square planar
  2. Trigonal bipyramidal Correct answer.
  3. Tetrahedral
  4. Octahedral

Explanation

Phosphorus in PF5 has five bonding pairs and no lone pairs, giving a total of five electron domains. Five domains arrange themselves into a trigonal bipyramidal geometry to minimise repulsion.

Question 35Chemical fundamentals: structure, properties and reactions

Which property best explains why graphite conducts electricity while diamond does not?

  1. Graphite contains mobile ions within its layered structure
  2. Graphite has delocalised electrons between its layers of hexagonal carbon sheets Correct answer.
  3. Diamond has a higher density than graphite
  4. Diamond contains weaker covalent bonds than graphite

Explanation

In graphite, each carbon is bonded to three others, leaving one electron per atom delocalised in a pi system that can move along the layers. Diamond has all four valence electrons locked in sigma bonds, so no mobile charge carriers exist.

Question 36Chemical fundamentals: structure, properties and reactions

Which statement about the reaction quotient Q and equilibrium constant K is correct for a system that is not at equilibrium?

  1. If Q is less than K, the reaction proceeds in the forward direction Correct answer.
  2. If Q equals K, the reaction has stopped completely
  3. If Q is less than K, the reaction proceeds in the reverse direction
  4. If Q is greater than K, the reaction proceeds in the forward direction

Explanation

The reaction quotient compares current concentrations with the equilibrium ratio. When Q is smaller than K, the forward reaction must increase products to raise Q toward K.

Question 37Chemical fundamentals: structure, properties and reactions

A catalyst is added to a reaction at equilibrium. Which statement describes the effect?

  1. The equilibrium constant increases because the reaction becomes faster
  2. The forward rate increases more than the reverse rate, shifting the equilibrium to the right
  3. The equilibrium position shifts to the left because the reverse reaction is favoured
  4. Both the forward and reverse rates increase equally, so the equilibrium position is unchanged Correct answer.

Explanation

A catalyst lowers the activation energy of both the forward and reverse reactions by the same amount. The rates increase equally, so equilibrium is reached faster but the position and value of K remain unchanged.

Question 38Chemical fundamentals: structure, properties and reactions

Which change would increase the amount of ammonia produced in the exothermic equilibrium N2(g) + 3H2(g) <-> 2NH3(g)?

  1. Increasing the temperature
  2. Increasing the volume of the container
  3. Adding a catalyst
  4. Decreasing the temperature Correct answer.

Explanation

The forward reaction is exothermic, so lowering the temperature favours the direction that releases heat, which is ammonia formation. The system shifts to the right to counteract the removal of heat.

Question 39Chemical fundamentals: structure, properties and reactions

In the reaction between hydrogen peroxide and iodide ions, the rate is found to be first order with respect to both reactants. What happens to the rate if the concentration of each reactant is doubled?

  1. The rate increases by a factor of two
  2. The rate increases by a factor of four Correct answer.
  3. The rate increases by a factor of eight
  4. The rate stays the same

Explanation

For a reaction first order in each of two reactants, the rate law is rate = k[A][B]. Doubling both concentrations multiplies the rate by 2 x 2 = 4.

Question 40Chemical fundamentals: structure, properties and reactions

Which species is the reducing agent in the reaction 2FeCl3(aq) + SnCl2(aq) -> 2FeCl2(aq) + SnCl4(aq)?

  1. FeCl3
  2. FeCl2
  3. SnCl4
  4. SnCl2 Correct answer.

Explanation

Tin in SnCl2 is oxidised from +2 to +4, so it donates electrons and is the reducing agent. The iron species is reduced from +3 to +2 and acts as the oxidising agent.

Question 41Chemical fundamentals: structure, properties and reactions

Which statement correctly describes the sign of the entropy change for the sublimation of solid carbon dioxide?

  1. The entropy change is negative because energy is absorbed from the surroundings
  2. The entropy change is zero because the substance does not change chemically
  3. The entropy change is positive because gas molecules are far more disordered than the solid Correct answer.
  4. The entropy change is negative because the substance becomes more ordered

Explanation

Sublimation converts a highly ordered solid into a gas, in which particles move freely and occupy many more microstates. The increase in disorder means the entropy change is positive.

Question 42Chemical fundamentals: structure, properties and reactions

Which statement about the spontaneity of a reaction is correct?

  1. A reaction is spontaneous only when its enthalpy change is negative
  2. A reaction is spontaneous whenever its activation energy is low
  3. A reaction is spontaneous when the total entropy change of the system and surroundings is positive Correct answer.
  4. A reaction is spontaneous only when the entropy change of the system is positive

Explanation

The second law states that the total entropy change of the universe, the sum of system and surroundings, must be positive for a process to be spontaneous. Considering only the system's entropy or enthalpy change is insufficient.

Question 43Chemical fundamentals: structure, properties and reactions

A galvanic cell is constructed with a magnesium electrode in Mg(NO3)2 solution and a copper electrode in Cu(NO3)2 solution. Given E°(Mg2+/Mg) = -2.37 V and E°(Cu2+/Cu) = +0.34 V, what is the standard cell potential?

  1. -2.03 V
  2. +2.03 V
  3. +2.71 V Correct answer.
  4. +1.18 V

Explanation

The standard cell potential equals the reduction potential of the cathode minus that of the anode. Copper is the cathode and magnesium the anode, giving 0.34 - (-2.37) = 2.71 V.

Question 44Chemical fundamentals: structure, properties and reactions

Which statement correctly compares the strength of a weak monoprotic acid with that of a strong monoprotic acid at the same concentration?

  1. Both acids have the same pH because their concentrations are equal
  2. The weak acid has a smaller acid dissociation constant and a higher pH Correct answer.
  3. The weak acid produces more hydronium ions per molecule than the strong acid
  4. The weak acid has a larger acid dissociation constant and a lower pH

Explanation

A weak acid dissociates only partially, so its acid dissociation constant is small and the equilibrium hydronium concentration is lower. A lower hydronium concentration corresponds to a higher pH.

Question 45Chemical fundamentals: structure, properties and reactions

A 0.10 mol/L solution of a weak acid has a pH of 3.0. What is the approximate acid dissociation constant?

  1. 1 x 10^-1
  2. 1 x 10^-7
  3. 1 x 10^-5 Correct answer.
  4. 1 x 10^-3

Explanation

A pH of 3.0 means [H+] equals 1 x 10^-3 mol/L. For a weak acid, Ka is approximately [H+]^2 divided by the initial concentration, giving (1 x 10^-3)^2 / 0.10 = 1 x 10^-5.

Question 46Chemical fundamentals: structure, properties and reactions

Which statement correctly describes the role of a buffer solution made from ethanoic acid and sodium ethanoate?

  1. It prevents any change in pH regardless of how much acid or base is added
  2. It resists pH change because the acid neutralises added base and the conjugate base neutralises added acid Correct answer.
  3. It maintains a fixed pH because it contains equal amounts of strong acid and strong base
  4. It works by diluting any added acid or base to a negligible concentration

Explanation

A buffer contains a weak acid and its conjugate base in appreciable amounts, so each component can consume added hydroxide or hydronium ions. This keeps the pH nearly constant until one component is largely used up.

Question 47Chemical fundamentals: structure, properties and reactions

Which pair of species would act as a buffer when mixed in comparable amounts?

  1. HCl and NaCl
  2. NaOH and NaCl
  3. H2CO3 and HCO3- Correct answer.
  4. H2CO3 and NaCl

Explanation

A buffer requires a weak acid and its conjugate base in appreciable concentrations. Carbonic acid and its conjugate base hydrogen carbonate fit this requirement.

Question 48Chemical fundamentals: structure, properties and reactions

A sample of gas occupies 250 mL at 25 °C and 100 kPa. What volume would it occupy at 25 °C and 50 kPa, assuming ideal behaviour?

  1. 125 mL
  2. 250 mL
  3. 1000 mL
  4. 500 mL Correct answer.

Explanation

At constant temperature, Boyle's law states that pressure and volume are inversely proportional, so P1V1 = P2V2. Halving the pressure from 100 to 50 kPa doubles the volume from 250 mL to 500 mL.

Question 49Chemical fundamentals: structure, properties and reactions

Which sample contains the same number of molecules as 2.0 mol of oxygen gas, O2?

  1. 0.5 mol of nitrogen gas, N2
  2. 4.0 mol of helium, He
  3. 1.0 mol of carbon dioxide, CO2
  4. 2.0 mol of carbon dioxide, CO2 Correct answer.

Explanation

The number of molecules depends only on the amount in moles, since one mole of any substance contains Avogadro's number of particles. Equal moles of O2 and CO2 therefore contain equal numbers of molecules.

Question 50Chemical fundamentals: structure, properties and reactions

Which statement best explains why the first ionisation energy of sodium is lower than that of magnesium?

  1. Sodium has a larger nuclear charge than magnesium, which attracts its outer electron more strongly
  2. Magnesium has a full outer shell, making its electron harder to remove
  3. Sodium's outer electron is in the 3s orbital and is shielded by the filled 2p level, while magnesium has a higher effective nuclear charge Correct answer.
  4. Sodium has a larger number of electron shells than magnesium, so its outer electron is further from the nucleus

Explanation

Both elements lose an electron from the third shell, but magnesium has one more proton and the same shielding, giving a higher effective nuclear charge. The outer electron in sodium is therefore held less tightly and requires less energy to remove.