Chemistry questions and solutions

Browse original practice and official past-paper questions. Each question has its own link, with its source and worked solution.

364 questions · Page 13 of 16

1. Q80 · Original practice · 5 marks
   The Haber reaction is $\mathrm{N_2(g)+3H_2(g)\rightleftharpoons2NH_3(g)}$, with $\Delta H<0$. A designer considers lowering the operating temperature from 450 °C to 100 °C while keeping the pressure fixed.
   Chemical synthesis
2. Q81 · Original practice · 6 marks
   In the contact process, $\mathrm{2SO_2(g)+O_2(g)\rightleftharpoons2SO_3(g)}$. A feed contains 8.00 mol SO₂ and 3.00 mol O₂.
   Chemical synthesis
3. Q82 · Original practice · 5 marks
   A student reacts 0.300 mol ethanol with 0.200 mol ethanoic acid to make ethyl ethanoate. The isolated ester mass is 15.0 g. Use $M(\text{ester})=88.1\ \mathrm{g\,mol^{-1}}$.
   Chemical synthesis
4. Q83 · Original practice · 6 marks
   A fermentation consumes 36.0 g glucose and produces 14.7 g ethanol. Use $M(\text{glucose})=180.0$ and $M(\text{ethanol})=46.0\ \mathrm{g\,mol^{-1}}$.
   Chemical synthesis
5. Q84 · Original practice · 4 marks
   Tetrafluoroethene, $\mathrm{CF_2=CF_2}$, forms PTFE.
   Polymers and biomolecules
6. Q85 · Original practice · 5 marks
   A 20.00 mL sample of an unknown weak monoprotic acid requires 15.00 mL of 0.1200 mol L⁻¹ NaOH. The untreated acid has pH 2.80.
   Acids, bases and titration
7. Q86 · Original practice · 7 marks
   A saturated solution of $\mathrm{Mg(OH)_2}$ is filtered to remove all undissolved solid. At $25^\circ\mathrm C$, $K_{sp}=1.8\times10^{-11}$ and $K_w=1.0\times10^{-14}$. Assume dissolution is the only significant source of hydroxide.
   Chemical equilibrium
8. Q87 · Original practice · 6 marks
   50.0 mL of 0.100 mol L⁻¹ AgNO₃ is mixed with 50.0 mL of 0.100 mol L⁻¹ NaCl at $25^\circ\mathrm C$. AgCl precipitates. For AgCl, $K_{sp}=1.8\times10^{-10}$. After equilibrium, the liquid is completely separated from the solid. Use $F=96485\ \mathrm{C\,mol^{-1}}$.
   Redox and electrochemistry
9. Q88 · Original practice · 7 marks
   A pure monoprotic carboxylic acid contains 40.0% C, 6.67% H and 53.3% O by mass. A 0.360 g sample is made up to 250.0 mL; a 25.00 mL aliquot requires 12.00 mL of 0.05000 mol L⁻¹ NaOH. The undiluted 250.0 mL acid solution has pH 3.19. Use C = 12.0, H = 1.0, O = 16.0.
   Organic analysis
10. Q89 · Original practice · 7 marks
    A copper-plating cell runs at 1.50 A for 32.0 min. It deposits 0.740 g Cu. All current not used for copper deposition produces hydrogen by $\mathrm{2H^++2e^-\to H_2}$. Use $F=96485\ \mathrm{C\,mol^{-1}}$, $M(Cu)=63.55\ \mathrm{g\,mol^{-1}}$ and gas molar volume $24.8\ \mathrm{L\,mol^{-1}}$.
    Redox and electrochemistry
11. Q90 · Original practice · 6 marks
    A pathway starts from 2-bromo-2-methylbutane. Heated ethanolic hydroxide gives predominantly 2-methylbut-2-ene. This alkene undergoes acid-catalysed hydration, giving the Markovnikov alcohol.
    Organic chemistry
12. Q91 · Original practice · 6 marks
    An acidic hydrogen–oxygen fuel cell supplies 2.00 A for 1.00 h. Use $F=96485\ \mathrm{C\,mol^{-1}}$ and gas molar volume $24.8\ \mathrm{L\,mol^{-1}}$. Exactly 50.0% of the hydrogen fed into the cell reacts; assume all current comes from hydrogen oxidation.
    Chemical synthesis
13. Q92 · Original practice · 6 marks
    Ethanoic acid reacts with ethanol in a non-aqueous homogeneous mixture of constant volume 100.0 mL. Initially each reactant has concentration 0.100 mol L⁻¹ and neither product is present. For this model, $K_c=\dfrac{[\text{ester}][\mathrm{H_2O}]}{[\text{acid}][\text{ethanol}]}=4.00$. The ester molar mass is 88.1 g mol⁻¹.
    Chemical equilibrium
14. Q93 · Original practice · 6 marks
    A 2.00 mL hydrogen peroxide sample reacts completely with 20.00 mL of 0.05000 mol L⁻¹ Fe²⁺ in excess acid. Peroxide is reduced according to $\mathrm{H_2O_2+2H^++2e^-\to2H_2O}$ while Fe²⁺ forms Fe³⁺. The remaining Fe²⁺ requires 10.00 mL of 0.01000 mol L⁻¹ permanganate. Each mole of permanganate reacts with five moles Fe²⁺.
    Redox and electrochemistry
15. Q94 · Original practice · 6 marks
    A mixture initially contains 0.0100 mol L⁻¹ each of Cu²⁺ and Mg²⁺. Hydroxide is slowly added at $25^\circ\mathrm C$ with negligible volume change. $K_{sp}(\mathrm{Cu(OH)_2})=2.2\times10^{-20}$; $K_{sp}(\mathrm{Mg(OH)_2})=1.8\times10^{-11}$; $K_w=10^{-14}$. Ignore complex ions.
    Chemical equilibrium
16. Q95 · Original practice · 6 marks
    The ester shown undergoes acid-catalysed hydrolysis. Its alcohol product is then heated with acidified dichromate.
    Organic chemistry
17. Q96 · Original practice · 7 marks
    Two amino-acid standards have these properties: glutamic acid, $R_f=0.25$ and pI = 3.2; arginine, $R_f=0.65$ and pI = 10.8. A mixture on the same TLC plate gives spots 2.0 cm and 5.2 cm from a baseline; the solvent front travels 8.0 cm. A separate portion undergoes electrophoresis at pH 6.0.
    Organic analysis
18. Q97 · Original practice · 7 marks
    A zinc–copper galvanic cell contains 50.0 mL of 0.100 mol L⁻¹ Cu²⁺ solution and excess zinc. In an ideal model, Cu²⁺ reduction is the only cathode reaction. The cell supplies a constant 0.200 A until all initial Cu²⁺ is consumed. Use $F=96485\ \mathrm{C\,mol^{-1}}$, $M(Cu)=63.55$ and $M(Zn)=65.40\ \mathrm{g\,mol^{-1}}$.
    Redox and electrochemistry
19. Q98 · Original practice · 6 marks
    An unknown acyclic carbonyl compound is either butanal or butan-2-one. It has molecular-ion mass 72 and no broad O–H IR band. Heated acidified dichromate changes orange to green. Oxidation of 0.360 g of the unknown gives 0.400 g of the corresponding carboxylic acid. Use molar masses: unknown 72.1 g mol⁻¹; acid 88.1 g mol⁻¹.
    Organic analysis
20. Q99 · Original practice · 7 marks
    The energy profile shows a reversible reaction. Reactants have energy 40 kJ mol⁻¹, products 10 kJ mol⁻¹, and the uncatalysed transition state 110 kJ mol⁻¹. The dashed catalysed pathway peaks at 85 kJ mol⁻¹.
    Chemical equilibrium
21. Q100 · Original practice · 10 marks
    A miniature ammonia project generates hydrogen by electrolysis with a current of 2.00 A for 9648.5 s, at 100% efficiency. Hydrogen formation requires two electrons per molecule. The hydrogen then reacts with 0.0200 mol N₂ by $\mathrm{N_2+3H_2\to2NH_3}$. All ammonia produced is captured in 50.00 mL of 1.000 mol L⁻¹ HCl and the solution is made up to 100.0 mL.…
    Chemical synthesis
22. Q101 · Original practice · 1 mark
    Solid iodine and iodine vapour are present in a sealed tube at constant temperature. The colour of the vapour is constant. Which statement explains this observation?
    Chemical equilibrium
23. Q102 · Original practice · 1 mark
    A mixture is at equilibrium for $\mathrm{A(g)\rightleftharpoons 2B(g)}$. The vessel volume is instantly halved at constant temperature. Immediately after compression, before any net reaction, the reaction quotient is
    Chemical equilibrium
24. Q103 · Original practice · 1 mark
    Two weak monoprotic acids are compared at the same temperature. Assume that their measured hydrogen ions come from acid ionisation. Which conclusion is supported by the data?
    Acids, bases and titration