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Cameroon GCE A-Level Chemistry study materials
Cameroon GCE A-Level Chemistry study resources

Cameroon GCE A-Level Chemistry practice questions

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Question 1Atomic Structure and the Periodic Table

Which subatomic particle determines the atomic number of an element?

  1. The electron
  2. The neutron
  3. The proton Correct answer.
  4. The nucleon

Explanation

Atomic number is defined as the number of protons in the nucleus and is what identifies the element. Neutrons change the mass number but not the identity, and electrons can be gained or lost without changing the element.

Question 2Atomic Structure and the Periodic Table

An atom of an element has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁴. In which block and group of the Periodic Table does this element lie?

  1. s-block, group 2
  2. p-block, group 14
  3. p-block, group 16 Correct answer.
  4. d-block, group 16

Explanation

The last electrons enter a p subshell, so the element is in the p-block. The outer shell holds 2 + 4 = 6 electrons, which places it in group 16 (the oxygen group).

Question 3Atomic Structure and the Periodic Table

Which statement correctly describes the trend in first ionisation energy across a period from left to right?

  1. It increases because nuclear charge rises while shielding stays roughly constant. Correct answer.
  2. It increases because the atoms gain more electron shells.
  3. It stays constant because outer electrons feel the same effective nuclear charge.
  4. It decreases because atomic radius increases.

Explanation

Across a period the number of protons increases while electrons enter the same shell, so shielding hardly changes. The resulting rise in effective nuclear charge pulls the outer electron more strongly, raising the ionisation energy.

Question 4Atomic Structure and the Periodic Table

Which pair of species are isotopes of the same element?

  1. ³⁵Cl and ³⁷Cl Correct answer.
  2. ¹⁴N and ¹⁴C
  3. ²³Na and ²⁴Mg
  4. ¹⁶O and ¹⁶O²⁻

Explanation

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. ³⁵Cl and ³⁷Cl both have 17 protons but 18 and 20 neutrons respectively.

Question 5Atomic Structure and the Periodic Table

How many orbitals are present in a complete 3p subshell?

  1. Six
  2. Five
  3. One
  4. Three Correct answer.

Explanation

A p subshell contains three orbitals (px, py and pz), each able to hold two electrons, giving a maximum of six electrons. The number of orbitals is fixed by the value of the azimuthal quantum number.

Question 6Atomic Structure and the Periodic Table

Which property increases down group 2 of the Periodic Table?

  1. First ionisation energy
  2. Atomic radius Correct answer.
  3. Melting point
  4. Electronegativity

Explanation

Going down group 2, an extra electron shell is added at each step, so the atoms become larger. Ionisation energy and electronegativity fall as the outer electrons are further from the nucleus and better shielded.

Question 7Atomic Structure and the Periodic Table

A sample of magnesium contains 79% of ²⁴Mg, 10% of ²⁵Mg and 11% of ²⁶Mg. What is the relative atomic mass of the sample to three significant figures?

  1. 25.3
  2. 24.3 Correct answer.
  3. 24.0
  4. 25.0

Explanation

The relative atomic mass is the weighted mean: (0.79 × 24) + (0.10 × 25) + (0.11 × 26) = 18.96 + 2.50 + 2.86 = 24.32, which rounds to 24.3.

Question 8Atomic Structure and the Periodic Table

Which explanation best accounts for the decrease in first ionisation energy between magnesium and aluminium?

  1. Aluminium has a lower atomic mass than magnesium, weakening the attraction.
  2. Aluminium has a smaller nuclear charge than magnesium, so its outer electron is held less tightly.
  3. Aluminium atoms have one more electron shell than magnesium atoms.
  4. Aluminium has a larger nuclear charge but its outer electron is in a 3p orbital, slightly higher in energy and better shielded than the 3s electron of magnesium. Correct answer.

Explanation

Although aluminium has a higher nuclear charge, its outermost electron occupies a 3p orbital, which lies slightly higher in energy and is shielded by the filled 3s subshell. This outweighs the nuclear charge increase, so the ionisation energy falls.

Question 9Atomic Structure and the Periodic Table

Which species has the electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶?

  1. K⁺ Correct answer.
  2. S²⁻
  3. Ar⁺
  4. Cl⁻

Explanation

The configuration shown has 18 electrons and ends at 3p⁶, which is the argon arrangement. A potassium atom (19 electrons) losing one electron gives K⁺ with exactly this configuration.

Question 10Atomic Structure and the Periodic Table

Which statement about the shapes of atomic orbitals is correct?

  1. A d orbital is spherical and a p orbital is dumbbell-shaped.
  2. An s orbital is spherical and a p orbital is dumbbell-shaped. Correct answer.
  3. Both s and p orbitals are spherical but differ in size.
  4. An s orbital is dumbbell-shaped and a p orbital is spherical.

Explanation

The s orbital has no angular dependence and is spherical about the nucleus. The three p orbitals each have two lobes on opposite sides of the nucleus, giving the familiar dumbbell shape.

Question 11Atomic Structure and the Periodic Table

Why do the elements of period 3 show a general increase in electronegativity from sodium to chlorine?

  1. The atoms lose electrons more readily, increasing their electronegativity.
  2. Nuclear charge increases while the outer shell stays the same, so the nucleus attracts bonding electrons more strongly. Correct answer.
  3. The atomic radius increases, allowing closer approach of bonding electrons.
  4. The number of electron shells increases, drawing electrons in more strongly.

Explanation

Electronegativity measures the ability of an atom to attract electrons in a bond. Across period 3 the proton count rises while electrons occupy the same shell, so effective nuclear charge increases and the attraction for bonding electrons grows.

Question 12Atomic Structure and the Periodic Table

Which ion would be deflected least in a mass spectrometer when all ions carry the same charge?

  1. ²⁴Mg⁺
  2. ²⁵Mg⁺
  3. ²⁶Mg⁺ Correct answer.
  4. ²³Na⁺

Explanation

For ions of equal charge, the radius of deflection is inversely proportional to mass. The heaviest ion therefore follows the least curved path, and ²⁶Mg⁺ has the greatest mass among the options.

Question 13Atomic Structure and the Periodic Table

An element has successive ionisation energies that rise steadily, with a very large jump after the fourth ionisation energy. To which group does the element most likely belong?

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

Explanation

A large jump occurs when an electron is removed from a new, inner shell that is closer to the nucleus. If the jump follows the fourth ionisation, the atom had four outer electrons, which indicates group 4.

Question 14Atomic Structure and the Periodic Table

Which pair of elements would be expected to show the most similar chemical properties?

  1. Lithium and beryllium
  2. Fluorine and neon
  3. Sodium and magnesium
  4. Lithium and sodium Correct answer.

Explanation

Elements in the same group have the same number of outer-shell electrons and therefore similar chemical behaviour. Lithium and sodium are both group 1 alkali metals.

Question 15Atomic Structure and the Periodic Table

Which isoelectronic series is arranged in order of increasing ionic radius?

  1. Na⁺ < Mg²⁺ < F⁻ < O²⁻
  2. F⁻ < O²⁻ < Na⁺ < Mg²⁺
  3. O²⁻ < F⁻ < Na⁺ < Mg²⁺
  4. Mg²⁺ < Na⁺ < F⁻ < O²⁻ Correct answer.

Explanation

All four ions have the neon configuration of 10 electrons. For isoelectronic species, radius decreases as nuclear charge increases, so the order from smallest to largest is Mg²⁺ (12 protons), Na⁺ (11), F⁻ (9) then O²⁻ (8).

Question 16Atomic Structure and the Periodic Table

The first ionisation energy of neon is much higher than that of sodium even though sodium has a higher nuclear charge. Which explanation is correct?

  1. Neon's outer electron is in a lower energy shell and is removed from a stable full octet, whereas sodium's single 3s electron is well shielded and easily lost. Correct answer.
  2. Sodium has a lower nuclear charge, so its electron is held more tightly.
  3. Neon has more electron shells, providing extra shielding.
  4. Sodium's outer electron is in a p orbital, which is harder to remove than neon's s electron.

Explanation

Neon's outermost electrons complete the 2p subshell, and removal breaks a stable octet close to an unshielded nucleus, so the ionisation energy is very high. Sodium's single 3s electron lies in a new shell, is strongly shielded by the neon core and is lost easily.

Question 17Atomic Structure and the Periodic Table

Copper has the electron configuration [Ar] 3d¹⁰ 4s¹ rather than [Ar] 3d⁹ 4s². Which explanation is most consistent with this observation?

  1. The 4s orbital is always lower in energy than the 3d orbitals, so it must fill first.
  2. Copper has one fewer electron than zinc, forcing an electron out of the 3d subshell.
  3. A completely filled 3d subshell with a half-filled 4s orbital gives a more stable arrangement, and the energy gap between 3d and 4s is small. Correct answer.
  4. Electrons always pair in the same orbital before entering a new subshell.

Explanation

The 3d and 4s energies are very close in the transition elements, so small stability gains matter. A full 3d subshell together with a half-filled 4s orbital is more stable than the alternative, which is why copper adopts this configuration.

Question 18Atomic Structure and the Periodic Table

An element X forms a chloride with the formula XCl₃ and its highest-energy electrons are in the third shell. Which statement about X is correct?

  1. X is in group 13 and its atoms have three outer-shell electrons. Correct answer.
  2. X is in group 15 and forms XCl₃ by gaining three electrons.
  3. X is a transition metal because its outer electrons are in the third shell.
  4. X is in group 3 and its atoms have three electron shells.

Explanation

The formula XCl₃ shows that X commonly forms three bonds, consistent with three outer-shell electrons in group 13. Having outer electrons in the third shell simply places it in period 3.

Question 19Atomic Structure and the Periodic Table

Which observation from the mass spectrum of a diatomic element provides the strongest evidence for the existence of isotopes?

  1. A peak corresponding to doubly charged ions at half the mass-to-charge ratio
  2. A base peak produced by fragmentation of the molecule
  3. A single molecular ion peak at the expected mass
  4. Two molecular ion peaks separated by one mass unit in roughly equal intensity Correct answer.

Explanation

Two molecular ion peaks differing by one unit show molecules of the same element with different numbers of neutrons. This is direct evidence for isotopes because the chemical identity is unchanged while the mass differs.

Question 20Atomic Structure and the Periodic Table

Which factor best explains why the atomic radius of chlorine is smaller than that of bromine?

  1. Chlorine has a full outer shell, which contracts the atom.
  2. Bromine has fewer protons, so its electrons are pulled in more weakly.
  3. Chlorine's outer electrons are in a shell closer to the nucleus, with fewer inner shells shielding them. Correct answer.
  4. Chlorine has a greater nuclear charge than bromine.

Explanation

Down a group an extra electron shell is added at each step, so the outer electrons lie further from the nucleus and are more shielded. Chlorine, being above bromine, has its outer electrons in the third shell rather than the fourth, giving a smaller radius.

Question 21Chemical Bonding and Structure

Which type of bonding is present in solid sodium chloride?

  1. Hydrogen bonding between Na⁺ and Cl⁻ ions
  2. Covalent bonding between Na and Cl atoms
  3. Metallic bonding between Na⁺ ions and delocalised electrons
  4. Ionic bonding between Na⁺ and Cl⁻ ions Correct answer.

Explanation

Sodium chloride is a giant ionic lattice in which each sodium atom transfers one electron to chlorine, forming Na⁺ and Cl⁻ ions held together by strong electrostatic attraction in all directions. There are no shared electron pairs between Na and Cl, and no free electrons as in a metal.

Question 22Chemical Bonding and Structure

Which property is characteristic of a simple molecular substance such as iodine, I₂?

  1. It has a high melting point because of giant covalent bonds
  2. It has a low melting point because only weak intermolecular forces are overcome Correct answer.
  3. It dissolves readily in water to give conducting solutions
  4. It conducts electricity when molten

Explanation

Simple molecular solids are held in the lattice by weak van der Waals forces, so little energy is needed to separate the molecules and melting points are low. The strong covalent bonds are intramolecular and are not broken on melting.

Question 23Chemical Bonding and Structure

What is the shape of a methane molecule, CH₄, according to VSEPR theory?

  1. Square planar
  2. Trigonal pyramidal
  3. Bent
  4. Tetrahedral Correct answer.

Explanation

Carbon in methane has four bonding pairs and no lone pairs, and the four pairs repel each other equally to positions of maximum separation. This gives bond angles of about 109.5° and a regular tetrahedral shape.

Question 24Chemical Bonding and Structure

Which statement about a dative covalent (coordinate) bond is correct?

  1. The bond is weaker than a normal covalent bond
  2. Each atom contributes one electron to the shared pair
  3. Both shared electrons come from the same atom Correct answer.
  4. The bond can form only between ions of opposite charge

Explanation

In a dative covalent bond the shared electron pair is donated entirely by one atom, which must have a lone pair, to another atom with an empty orbital. A classic example is the N→B bond in ammonia–boron trifluoride, H₃N→BF₃.

Question 25Chemical Bonding and Structure

Which of the following substances has a giant covalent (macromolecular) structure?

  1. Sodium chloride
  2. Silicon dioxide Correct answer.
  3. Iodine
  4. Carbon dioxide

Explanation

Silicon dioxide consists of a three-dimensional network of silicon and oxygen atoms joined by covalent bonds, giving a very high melting point and hardness. Carbon dioxide and iodine are simple molecular, while sodium chloride is ionic.

Question 26Chemical Bonding and Structure

Which pair of elements would be most likely to form an ionic compound when combined?

  1. Hydrogen and sulfur
  2. Nitrogen and chlorine
  3. Potassium and fluorine Correct answer.
  4. Carbon and oxygen

Explanation

Ionic bonding occurs between a metal of low ionisation energy and a non-metal of high electron affinity, so potassium (a Group 1 metal) and fluorine (a very electronegative non-metal) form KF. Large differences in electronegativity favour electron transfer rather than sharing.

Question 27Chemical Bonding and Structure

Why does magnesium have a higher melting point than sodium?

  1. Magnesium has more delocalised electrons per atom and a higher nuclear charge, giving stronger metallic bonding Correct answer.
  2. Magnesium forms covalent bonds in the solid state
  3. Magnesium has a lower charge density on its cation
  4. Magnesium atoms are larger than sodium atoms

Explanation

Each magnesium atom contributes two electrons to the delocalised sea, and the Mg²⁺ ion has a higher charge than Na⁺, so electrostatic attraction within the lattice is stronger. More energy is therefore needed to melt magnesium.

Question 28Chemical Bonding and Structure

The bond angle in a water molecule is approximately 104.5°, whereas the tetrahedral angle is 109.5°. What explains this difference?

  1. Water is a linear molecule with two separate dipoles
  2. The oxygen–hydrogen bonds are ionic
  3. Hydrogen atoms repel one another strongly
  4. Two lone pairs on oxygen repel the bonding pairs more strongly than bonding pairs repel each other Correct answer.

Explanation

Oxygen in water has two bonding pairs and two lone pairs. Lone-pair–lone-pair repulsion is greatest, followed by lone-pair–bonding-pair repulsion, so the H–O–H angle is compressed below the tetrahedral value to about 104.5°.

Question 29Chemical Bonding and Structure

Which molecule is polar overall?

  1. Ammonia, NH₃ Correct answer.
  2. Boron trifluoride, BF₃
  3. Carbon tetrachloride, CCl₄
  4. Carbon dioxide, CO₂

Explanation

Ammonia has a trigonal pyramidal shape with a lone pair on nitrogen, so the three N–H bond dipoles do not cancel and the molecule has a net dipole moment. In the other molecules the individual bond dipoles cancel because of their symmetrical geometry.

Question 30Chemical Bonding and Structure

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

  1. Each carbon atom in graphite is bonded to three others, leaving one delocalised electron per atom Correct answer.
  2. Diamond contains only ionic bonds that hold electrons in place
  3. Graphite has a lower melting point than diamond
  4. Graphite contains mobile ions between its layers

Explanation

In graphite each carbon is bonded to three others in a hexagonal layer and the fourth valence electron is delocalised across the layer, allowing conduction. In diamond all four valence electrons are used in sigma bonds within a rigid tetrahedral network, so no electrons are free to move.

Question 31Chemical Bonding and Structure

Which intermolecular force is mainly responsible for the relatively high boiling point of ethanol compared with ethane?

  1. Ionic attraction between molecules
  2. Hydrogen bonding involving the O–H group Correct answer.
  3. Covalent bonding between separate ethanol molecules
  4. Instantaneous dipole–induced dipole forces

Explanation

Ethanol contains an O–H group, so hydrogen bonds form between the hydrogen of one molecule and the lone pair on the oxygen of another. These are much stronger than the van der Waals forces present in ethane, which has no O–H group.

Question 32Chemical Bonding and Structure

Which species has a bond angle of 180°?

  1. CO₂ Correct answer.
  2. NH₄⁺
  3. H₃O⁺
  4. SO₂

Explanation

In carbon dioxide the central carbon has two bonding regions and no lone pairs, and VSEPR theory places two electron domains at 180° to minimise repulsion. The molecule is therefore linear with an O=C=O angle of 180°.

Question 33Chemical Bonding and Structure

A solid has a high melting point, does not conduct electricity when solid, but conducts when molten. Which type of structure does it most likely have?

  1. Giant covalent
  2. Metallic
  3. Simple molecular
  4. Giant ionic Correct answer.

Explanation

Ionic solids have strong electrostatic attractions throughout the lattice, so melting points are high, and the ions are fixed in the solid so it does not conduct. Once molten, the ions become mobile and carry charge, giving conductivity.

Question 34Chemical Bonding and Structure

Which of the following molecules contains a dative covalent bond?

  1. HCl
  2. NH₄⁺ Correct answer.
  3. CH₄
  4. H₂O

Explanation

The ammonium ion forms when the lone pair on ammonia is donated to a proton that has an empty 1s orbital, creating one dative covalent N–H bond. Once formed it is identical to the other three N–H bonds.

Question 35Chemical Bonding and Structure

Which statement about the electrical conductivity of metals is correct?

  1. Conduction occurs only when the metal is molten
  2. Conduction occurs by the movement of delocalised valence electrons Correct answer.
  3. Conduction requires the breaking of covalent bonds in the lattice
  4. Conduction occurs by the movement of positive metal ions

Explanation

In a metal the valence electrons are delocalised and free to move through the lattice of cations, so a potential difference causes electron flow. This conduction persists in the solid state and does not require bond breaking.

Question 36Chemical Bonding and Structure

Which property would you expect for a substance with a giant covalent network structure such as silicon carbide?

  1. It is hard and has a very high melting point Correct answer.
  2. It conducts electricity well in the solid state
  3. It dissolves readily in water to give a conducting solution
  4. It is a soft solid with a low melting point

Explanation

In a giant covalent network every atom is held by strong covalent bonds extending throughout the structure, so large amounts of energy are needed to break the lattice. This gives high hardness and very high melting points.

Question 37Chemical Bonding and Structure

Which of the following correctly lists the substances in order of increasing melting point?

  1. NaCl < CH₄ < MgO
  2. CH₄ < MgO < NaCl
  3. CH₄ < NaCl < MgO Correct answer.
  4. MgO < NaCl < CH₄

Explanation

Methane is a simple molecular solid with only weak dispersion forces, so it melts at a very low temperature. Sodium chloride is ionic but MgO has ions of higher charge (2+ and 2−), so its lattice energy and melting point are greater than those of NaCl.

Question 38Chemical Bonding and Structure

Predict the shape and bond angle of the BF₄⁻ ion.

  1. Tetrahedral, 109.5° Correct answer.
  2. Square planar, 90°
  3. Trigonal pyramidal, 107°
  4. Linear, 180°

Explanation

The tetrafluoroborate ion has a central boron atom bonded to four fluorines with no lone pairs, giving four electron domains that arrange tetrahedrally. All four B–F bonds are equivalent with angles of 109.5°.

Question 39Chemical Bonding and Structure

Why is the first ionisation energy of magnesium higher than that of sodium?

  1. Magnesium has a larger atomic radius than sodium
  2. Sodium has a more stable noble-gas configuration
  3. Magnesium has a greater nuclear charge with the outer electron in the same shell, so it is held more strongly Correct answer.
  4. Magnesium has an extra electron shell shielding the outer electron

Explanation

Across Period 3 the outer electrons remain in the 3s subshell while nuclear charge increases from +11 to +12, so the effective nuclear attraction on the outer electron is greater in magnesium. The outer electron is therefore removed less easily from sodium than from magnesium.

Question 40Chemical Bonding and Structure

Which observation provides the best evidence that the bonding in a metal is not directional covalent bonding?

  1. Metals can be hammered into sheets without shattering Correct answer.
  2. Metals are shiny when freshly cut
  3. Metals have high densities
  4. Metals react with acids to give hydrogen

Explanation

If bonding were directional and fixed, deformation would break the bonds and the solid would fracture. The ability of metals to deform without breaking shows that the delocalised electron cloud allows layers of cations to slide while the metallic bonding is maintained.

Question 41States of Matter, Gases and Kinetic Theory

A fixed mass of gas at constant temperature has its volume halved. According to Boyle's law, what happens to its pressure?

  1. It is halved.
  2. It increases fourfold.
  3. It remains unchanged.
  4. It is doubled. Correct answer.

Explanation

Boyle's law states that at constant temperature the pressure of a fixed mass of gas is inversely proportional to its volume, so pV is constant. Halving the volume therefore doubles the pressure. The relationship holds only because the temperature and amount of gas are unchanged.

Question 42States of Matter, Gases and Kinetic Theory

Which statement is part of the kinetic theory model of an ideal gas?

  1. Collisions between gas particles are perfectly elastic. Correct answer.
  2. Gas particles attract one another strongly except during collisions.
  3. The average kinetic energy of the particles depends on the pressure of the gas.
  4. The volume occupied by the particles themselves is a large fraction of the container volume.

Explanation

The kinetic theory assumes that collisions between particles and with the walls are perfectly elastic, so no kinetic energy is lost overall. It also assumes negligible intermolecular forces and negligible particle volume compared with the container. The average kinetic energy of the particles depends on the absolute temperature, not on pressure.

Question 43States of Matter, Gases and Kinetic Theory

A sample of gas occupies 300 cm³ at 27 °C and 1.0 atm. What volume would it occupy at 127 °C and the same pressure?

  1. 700 cm³
  2. 300 cm³
  3. 400 cm³ Correct answer.
  4. 225 cm³

Explanation

Charles's law gives V₁/T₁ = V₂/T₂ at constant pressure, with temperatures converted to kelvin: 300 K and 400 K. So V₂ = 300 × (400/300) = 400 cm³. Using Celsius temperatures directly would give a wrong answer, since the gas laws require absolute temperature.

Question 44States of Matter, Gases and Kinetic Theory

Which property of a real gas is best explained by the presence of significant intermolecular forces between its particles?

  1. Its volume is directly proportional to absolute temperature at low pressure.
  2. It shows a measurable deviation from the ideal gas equation at high pressure and low temperature. Correct answer.
  3. Its pressure approaches zero as the temperature approaches absolute zero.
  4. Its particles move in straight lines between collisions.

Explanation

Real gases deviate from ideal behaviour most at high pressure and low temperature, where particles are close together. Under these conditions intermolecular attractions and the finite volume of particles become significant. The ideal gas equation ignores both effects, so deviations reveal them.

Question 45States of Matter, Gases and Kinetic Theory

Equal volumes of hydrogen and oxygen are kept in separate containers at the same temperature and pressure. Which statement is correct?

  1. The oxygen contains more molecules because its molecules are heavier.
  2. Both containers contain the same number of molecules. Correct answer.
  3. The number of molecules cannot be compared without knowing the container shape.
  4. The hydrogen contains more molecules because its molecules are lighter.

Explanation

Avogadro's law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules. The identity and mass of the gas molecules do not affect this. Molar mass affects the mass of gas present, not the number of molecules.

Question 46States of Matter, Gases and Kinetic Theory

A gaseous hydrocarbon contains 82.76 % carbon and 17.24 % hydrogen by mass. Its density at room temperature and pressure is about 2.59 g dm⁻³, taking the molar volume as 24.0 dm³ mol⁻¹. What is its molecular formula?

  1. C₃H₈
  2. C₂H₆
  3. C₄H₁₀ Correct answer.
  4. C₅H₁₂

Explanation

Dividing the percentages by atomic masses gives a C:H ratio of 6.897:17.24, which simplifies to 1:2.5, so the empirical formula is C₂H₅ with mass 29 g mol⁻¹. The molar mass from density is 2.59 × 24.0 ≈ 62 g mol⁻¹, exactly twice 29, so the molecular formula is C₄H₁₀.

Question 47States of Matter, Gases and Kinetic Theory

In the Maxwell–Boltzmann distribution for a gas, what happens to the curve when the temperature is raised?

  1. The curve shifts to higher speeds while keeping the same peak height.
  2. The total area under the curve decreases because more molecules escape.
  3. The peak becomes higher and shifts to lower molecular speeds.
  4. The peak becomes lower and broader and shifts to higher molecular speeds. Correct answer.

Explanation

Raising the temperature increases the average kinetic energy, so the distribution spreads over a wider range of higher speeds. Because the total number of molecules is fixed, the area under the curve stays constant and the peak must become lower. The most probable speed therefore increases.

Question 48States of Matter, Gases and Kinetic Theory

Which change increases the rate of diffusion of a gas through a porous membrane at constant temperature?

  1. Using a gas of lower molar mass. Correct answer.
  2. Increasing the pressure on both sides equally.
  3. Using a gas with stronger intermolecular attractions.
  4. Decreasing the temperature of the gas.

Explanation

Graham's law states that the rate of diffusion is inversely proportional to the square root of the molar mass. A gas of lower molar mass therefore diffuses faster at the same temperature. At a given temperature all gases have the same average kinetic energy, so lighter molecules move faster.

Question 49States of Matter, Gases and Kinetic Theory

A gas is collected over water at 25 °C, where the saturated vapour pressure of water is 3.2 kPa. The total pressure is 101.3 kPa and the volume collected is 250 cm³. How many moles of the dry gas are present? Take R = 8.31 J K⁻¹ mol⁻¹.

  1. About 0.0086 mol
  2. About 0.0099 mol Correct answer.
  3. About 0.0102 mol
  4. About 0.0122 mol

Explanation

The gas is saturated with water vapour, so its partial pressure is 101.3 − 3.2 = 98.1 kPa. Using n = pV/RT with p = 98 100 Pa, V = 2.50 × 10⁻⁴ m³ and T = 298 K gives n ≈ 0.0099 mol. Neglecting the water vapour pressure would overestimate the amount of dry gas.

Question 50States of Matter, Gases and Kinetic Theory

A vessel contains 4.0 g of helium and 32 g of oxygen at 300 K. Taking the molar masses as 4.0 and 32 g mol⁻¹, what is the mole fraction of helium?

  1. 0.89
  2. 0.25
  3. 0.11
  4. 0.50 Correct answer.

Explanation

The helium amount is 4.0/4.0 = 1.0 mol and the oxygen amount is 32/32 = 1.0 mol, giving 2.0 mol in total. The mole fraction of helium is therefore 1.0/2.0 = 0.50. Mole fraction depends on amounts in moles, not on masses.