Stoichiometry & Solutions
One mole of ethanoic acid, CH₃COOH, contains:
A) 6.022 × 10²³ atoms of carbon
B) 6.022 × 10²³ molecules
C) 6.022 × 10²³ atoms of oxygen
D) 6.022 × 10²³ atoms of hydrogen
B. Each molecule contains 2 C, 2 O and 4 H, so only the number of molecules equals Avogadro's number.
According to the Arrhenius theory, an acid is a substance that:
A) produces hydroxide ions in solution
B) produces (donates) hydrogen ions in solution
C) accepts protons in solution
D) turns red litmus blue
B
Which property of water cannot be explained by hydrogen bonding?
A) Its relatively low vapour pressure
B) Its relatively high melting point
C) Ice being less dense than liquid water
D) The bent (V-shaped) molecular shape of water
D. Molecular shape comes from VSEPR (2 bonding pairs and 2 lone pairs), not from intermolecular forces.
Students add equal masses of Na₂CO₃ (as a lump, small pieces and powder) to equal volumes of the same HCl solution, and time how long each takes to dissolve. What are the independent and dependent variables?
Independent: Particle Size, Dependent: Time to dissolve
Which of the following will increase the average kinetic energy of a gas?
A) Increasing the pressure of the gas
B) Decreasing the pressure of the gas
C) Increasing the volume of the container
D) Increasing the temperature of the gas
D. Average kinetic energy is proportional to absolute temperature.
What is the total concentration of all ions in 500 mL of 0.30 mol L⁻¹ Al(NO₃)₃ solution?
A) 0.30 mol L⁻¹
B) 0.60 mol L⁻¹
C) 0.90 mol L⁻¹
D) 1.2 mol L⁻¹
Al(NO₃)₃ → Al³⁺ + 3 NO₃⁻, so 0.30 + 0.90 = 1.2 mol L⁻¹. The volume doesn't matter.
Which of the following best describes a 10.00 mol L⁻¹ solution of H₃PO₄?
A) A concentrated solution of a strong acid
B) A dilute solution of a weak acid
C) A concentrated solution of a weak acid
D) A dilute solution of a weak base
C. "Concentrated" describes the amount of acid per litre, and "weak" means it only partially ionises.
Which of the following best explains why table salt (NaCl) is so soluble in water?
A) There are weak electrostatic attractions between the ions in NaCl(s)
B) Water is capable of hydrogen bonding
C) Ion–dipole forces form between water molecules and the Na⁺ and Cl⁻ ions
D) NaCl(s) has strong ionic bonding
C
C₂H₄(g) + H₂(g) → C₂H₆(g)
Which of the following would not increase the rate of this reaction?
A) Decreasing the volume of the reaction vessel
B) Increasing the temperature
C) Increasing the state of subdivision of the reactants
D) Adding a nickel catalyst
C. Both reactants are gases, so they're already fully "subdivided". Decreasing the volume increases concentration, so it does speed up the reaction.
Which of the following processes is exothermic?
A) Glaciers melting
B) Steam condensing on a bathroom mirror
C) A puddle evaporating in the sunshine
D) Water boiling in a kettle
B. Condensation releases energy as intermolecular forces form between the particles.
A balloon is filled with 7.1 L of helium gas at STP. What mass of helium has been added?
A) 0.31 g
B) 1.25 g
C) 1.78 g
D) 28.4 g
B. n = 7.1 / 22.71 = 0.3126 mol, and m = 0.3126 × 4.003 = 1.25 g
Which of the following solutions would have the highest pH?
A) 0.5 mol L⁻¹ HCl
B) 0.5 mol L⁻¹ HNO₃
C) 0.5 mol L⁻¹ H₂CO₃
D) 0.5 mol L⁻¹ H₂SO₄
C. H₂CO₃ is a weak acid, so it produces the lowest [H⁺] and therefore has the highest pH.
Boiling points: CH₃F −78.4 °C, CH₃Cl −24.2 °C, CH₃Br 3.56 °C, CH₃I 42.4 °C.
Identify the most polar compound and the compound with the strongest intermolecular forces.
F is the most electronegative, so CH₃F is the most polar. CH₃I has the largest molar mass, so it has the strongest dispersion forces and the highest boiling point. Polarity alone doesn't decide boiling point.
A small increase in temperature sometimes produces a large increase in reaction rate. The best explanation is:
A) The reaction must be endothermic
B) There is an increase in the rate of collisions between particles
C) A greater percentage of collisions have energy greater than the activation energy
D) The increased temperature lowers the activation energy
C.
B is true but only a minor effect, so it doesn't explain a large increase. D is a common misconception: temperature doesn't change the activation energy.
Which statement about an ideal gas is not correct?
A) 1 mole of an ideal gas occupies 22.71 L at STP
B) 1 mole of an ideal gas at absolute zero has a volume of 0 L
C) The particles of an ideal gas are weakly attracted to one another
D) The average kinetic energy of the particles is proportional to temperature
C. Ideal gas particles are assumed to have no attractive forces at all.
200.0 g of aluminium chloride, AlCl₃, is dissolved in water to make 700.0 mL of solution. Calculate the concentration of chloride ions to 3 s.f.
Answer: 6.43 mol L⁻¹.
n(AlCl₃) = 200.0 / 133.33 = 1.500 mol
n(Cl⁻) = 3 × 1.500 = 4.500 mol
c(Cl⁻) = 4.500 / 0.7000 = 6.43 mol L⁻¹
A 1.0 mol L⁻¹ HNO₃ solution conducts electricity much better than a 1.0 mol L⁻¹ NH₃ solution. The best explanation is that:
A) NH₃ is covalent molecular, while HNO₃ is ionic
B) NH₃ is a weak electrolyte, while HNO₃ is a strong electrolyte
C) NH₃ is basic, while HNO₃ is acidic
D) NH₃ ionises completely, while HNO₃ does not
B. HNO₃ ionises completely, which gives a higher concentration of mobile ions. (A is a common trap: HNO₃ is covalent molecular, and it ionises rather than dissociates.)
At 25 °C, NaCl has a solubility of 32 g per 100 mL. A 350 mL solution was found to contain 164 g of dissolved NaCl. The solution is best described as:
A) saturated
B) unsaturated
C) supersaturated
D) dilute
C. The maximum at 25 °C is 32 × 3.5 = 112 g, and 164 g is more than 112 g.
Manganese dioxide (MnO₂) is added to hydrogen peroxide as a catalyst. MnO₂ increases the reaction rate by:
A) increasing the average kinetic energy of the H₂O₂ particles
B) providing an alternative reaction pathway with a lower activation energy
C) making the enthalpy change (ΔH) more negative
D) reacting with H₂O₂ and being used up to form extra oxygen
B. A higher proportion of collisions now have enough energy to react. The catalyst isn't consumed, and ΔH doesn't change.
Reverse-phase HPLC uses a non-polar stationary phase and a polar mobile phase. On the chromatogram, prednisone comes out first (peak 1) and cortisol second (peak 2). Which inference is supported?
A) Prednisone is more polar than cortisol
B) Prednisone interacts more strongly with the stationary phase than cortisol
C) Cortisol has a lower molar mass than prednisone
D) Cortisol moves through the column more quickly than prednisone
A. Prednisone comes out first, so it's more attracted to the polar mobile phase, which means it's the more polar compound.
Hydrogen peroxide decomposes: 2 H₂O₂(l) → 2 H₂O(l) + O₂(g).
A 400 mL bottle of 6% w/v H₂O₂ contains 6 g of H₂O₂ per 100 mL. Calculate the volume of oxygen gas produced at STP.
Answer: 8.01 L.
m(H₂O₂) = 6 × 4 = 24 g
n(H₂O₂) = 24 / 34.016 = 0.7056 mol
n(O₂) = ½ × 0.7056 = 0.3528 mol
V(O₂) = 0.3528 × 22.71 = 8.01 L
Solid copper(II) carbonate is added to hydrochloric acid.
(a) Write the balanced ionic equation.
(b) State two observations.
(a) CuCO₃(s) + 2 H⁺(aq) → Cu²⁺(aq) + H₂O(l) + CO₂(g)
(b) Any two: the green powder dissolves; a blue solution forms; bubbles of a colourless gas are produced (effervescence).
Explain, in terms of intermolecular forces, why the boiling point of NCl₃ is higher than that of NH₃, even though NH₃ can hydrogen bond.
Answer (look for 3 of these 4 points):
NH₃ has dispersion, dipole–dipole and hydrogen bonding. NCl₃ has only dispersion and dipole–dipole forces.
NCl₃ has a much larger molar mass, so its dispersion forces are much stronger.
The total intermolecular forces in NCl₃ are therefore greater.
More energy is needed to separate NCl₃ molecules, so its boiling point is higher.
Explain, in terms of collision theory, why storing cut apple in the fridge slows down browning.
Answer (look for 3 of these 4 points):
A lower temperature decreases the average kinetic energy of the particles.
This decreases the frequency of collisions.
A smaller proportion of collisions have energy greater than or equal to the activation energy.
There are fewer successful collisions per second, so the rate of browning is slower.
A chemist fully vaporises 18.0 g of water and, separately, 18.0 g of liquid nitrogen. Both gases are stored under identical temperature and pressure. Which gas occupies the larger volume? Justify with a calculation.
The water vapour occupies the larger volume.
n(H₂O) = 18.0 / 18.016 = 1.00 mol
n(N₂) = 18.0 / 28.02 = 0.642 mol
At the same temperature and pressure, equal numbers of moles of any gas occupy equal volumes, so the gas with more moles (H₂O) takes up more space. At STP this is about 22.7 L compared with about 14.6 L.