1. 1
   A student carries out a titration to find the concentration of a hydrochloric acid solution. 25.0 cm³ of sodium hydroxide solution, of concentration 0.100 mol dm⁻³, is measured into a conical flask using a pipette, with a few drops of phenolphthalein indicator added. Hydrochloric acid is added from a burette until the indicator just changes colour, showing the end point has been reached. This exactly neutralises the sodium hydroxide when 20.0 cm³ of the hydrochloric acid has been added. The equation for the reaction is HCl + NaOH → NaCl + H₂O.

   (a)

   What is the amount, in moles, of sodium hydroxide used in the flask?

   [1 mark]

   • A2.50 × 10⁻³ mol
   • B2.50 × 10⁻² mol
   • C4.00 × 10⁻³ mol
   • D1.00 × 10⁻¹ mol

   (b)

   Which colour change is observed at the end point, as the hydrochloric acid is added from the burette into the sodium hydroxide solution containing phenolphthalein indicator?

   [1 mark]

   • Acolourless to pink
   • Bpink to colourless
   • Cblue to yellow
   • Dyellow to blue

   (c)

   Calculate the concentration of the hydrochloric acid, in mol dm⁻³, used in this titration.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student reacts 10.0 g of calcium carbonate (relative formula mass 100) with an excess of dilute hydrochloric acid, according to the equation CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The student evaporates and dries the resulting solution to obtain 3.85 g of solid calcium chloride (relative formula mass 111).

   (a)

   What is the theoretical yield of calcium chloride, in grams, for this reaction?

   [1 mark]

   • A3.85 g
   • B10.0 g
   • C11.1 g
   • D22.2 g

   (b)

   Using the theoretical yield of 11.1 g, which value is closest to the percentage yield of calcium chloride obtained by the student?

   [1 mark]

   • A34.7%
   • B3.85%
   • C65.3%
   • D11.1%

   (c)

   Suggest two reasons why the actual yield of calcium chloride obtained by the student is lower than the theoretical yield.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student reacts 0.0500 mol of magnesium ribbon with an excess of dilute sulfuric acid, according to the equation Mg + H₂SO₄ → MgSO₄ + H₂. All the hydrogen gas produced is collected and measured at room temperature and pressure, where the molar volume of any gas is 24 dm³ (24000 cm³).

   (a)

   Calculate the volume, in cm³, of hydrogen gas produced.

   [3 marks]

   (b)

   In a separate experiment, hydrogen gas is burned completely in oxygen gas to form water, according to the equation 2H₂ + O₂ → 2H₂O, with all volumes measured at the same temperature and pressure. Using Avogadro's law, calculate the volume, in cm³, of oxygen gas needed to react completely with 600 cm³ of hydrogen gas.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A company can manufacture ethanol by two different reaction pathways. Pathway 1 is fermentation, in which yeast enzymes convert glucose from plant sources into ethanol and carbon dioxide, at a moderate temperature and atmospheric pressure over several days in a batch process. Pathway 2 is the direct hydration of ethene with steam, using a phosphoric acid catalyst at 300°C and 60–70 atmospheres pressure, as a fast, continuous industrial process. Fermentation typically achieves a high percentage yield close to the theoretical maximum, but ethene for Pathway 2 is obtained from crude oil, a finite resource.

   (a)

   Explain the difference between percentage yield and atom economy, and explain, using the information given, why Pathway 2 has a higher atom economy than Pathway 1, even though Pathway 1 can achieve a higher percentage yield.

   [6 marks]

   (b)

   Evaluate, using the information given, whether Pathway 1 (fermentation) or Pathway 2 (direct hydration of ethene) is the better reaction pathway for the company to manufacture ethanol.

   [6 marks]

   Total for question 4: 12 marks

End of questions