Formulae, equations and amounts of substanceEdexcel A-Level Chemistry: Topic test

1. 1
   A single drop of pure water from a pipette has a mass of 0.090 g. Relative atomic masses: H = 1.0, O = 16.0. The Avogadro constant is 6.02 × 10²³ mol⁻¹.

   (a)

   What amount of water, in mol, is in the drop?

   [1 mark]

   • A5.00 × 10⁻³
   • B5.00 × 10⁻²
   • C1.62
   • D2.00 × 10²

   (b)

   How many hydrogen atoms are in the drop?

   [1 mark]

   • A3.01 × 10²¹
   • B9.03 × 10²¹
   • C6.02 × 10²¹
   • D6.02 × 10²²

   (c)

   Calculate the mass, in g, of one molecule of water. Give your answer in standard form to 3 significant figures.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A volatile liquid contains carbon, hydrogen and chlorine only. Analysis shows that it contains 24.3% carbon, 4.1% hydrogen and 71.6% chlorine by mass. In a separate experiment, 0.400 g of the liquid is vaporised and the vapour occupies 125 cm³ at 373 K and 100 kPa. Relative atomic masses: H = 1.0, C = 12.0, Cl = 35.5. The gas constant R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   What is the empirical formula of the liquid?

   [1 mark]

   • ACHCl
   • BCH₂Cl
   • CC₂H₄Cl₂
   • DCH₃Cl

   (b)

   Which expression gives the amount of vapour, in mol, using pV = nRT in SI units?

   [1 mark]

   • A(100 × 125) ÷ (8.31 × 373)
   • B(100 000 × 125) ÷ (8.31 × 373)
   • C(100 × 1.25 × 10⁻⁴) ÷ (8.31 × 373)
   • D(100 000 × 1.25 × 10⁻⁴) ÷ (8.31 × 373)

   (c)

   Calculate the molar mass of the liquid and deduce its molecular formula.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   25.0 cm³ of 0.200 mol dm⁻³ aqueous barium chloride is added to an excess of aqueous sodium sulfate. A precipitate forms, which is filtered off, washed and dried. Relative atomic masses: O = 16.0, S = 32.1, Ba = 137.3.

   (a)

   Write the ionic equation for the reaction, including state symbols, and state what is observed.

   [3 marks]

   (b)

   Calculate the maximum mass of precipitate that can form, in g, to 3 significant figures.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fertiliser works makes ammonium sulfate by passing ammonia gas into sulfuric acid: 2NH₃(g) + H₂SO₄(aq) → (NH₄)₂SO₄(aq). A technician neutralises exactly 250 cm³ of 0.500 mol dm⁻³ sulfuric acid. The molar volume of a gas under the conditions used is 24.0 dm³ mol⁻¹. The technician also checks the concentration of the acid by titrating 25.0 cm³ portions with 0.800 mol dm⁻³ sodium hydroxide, using methyl orange. The mean titre is 30.50 cm³ and each burette reading has an uncertainty of ±0.05 cm³. Relative atomic masses: H = 1.0, N = 14.0, O = 16.0, S = 32.1.

   (a)

   Calculate the volume of ammonia gas, in dm³, needed to neutralise the 250 cm³ of 0.500 mol dm⁻³ sulfuric acid, and the mass of ammonium sulfate formed.

   [6 marks]

   (b)

   Calculate the concentration of the sulfuric acid, in mol dm⁻³ and in g dm⁻³, from the titration results. Evaluate whether the technician's acid really has a concentration of 0.500 mol dm⁻³, using the percentage uncertainty in the mean titre.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Ethanol can be made by hydration of ethene, C₂H₄(g) + H₂O(g) → C₂H₅OH(g), or by fermentation of glucose, C₆H₁₂O₆(aq) → 2C₂H₅OH(aq) + 2CO₂(g). Relative atomic masses: H = 1.0, C = 12.0, O = 16.0.

   (a)

   What is the percentage atom economy of the fermentation of glucose to make ethanol?

   [1 mark]

   • A25.6%
   • B48.9%
   • C51.1%
   • D100%

   (b)

   Which statement about the hydration of ethene is correct?

   [1 mark]

   • AIts atom economy is 100%, because all the atoms in the reactants end up in the desired product.
   • BIts atom economy is less than 100%, because water is a reactant.
   • CIts percentage yield must be 100%, because its atom economy is 100%.
   • DIts atom economy cannot be calculated, because water is a liquid at room temperature.

   (c)

   In one process, 14.0 g of ethene gives 16.1 g of ethanol. Calculate the percentage yield.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   A student makes a standard solution by dissolving 1.325 g of anhydrous sodium carbonate, Na₂CO₃, in water and making it up to 250 cm³ in a volumetric flask. Hydrochloric acid is added from a burette to 25.0 cm³ portions of the solution until all the carbonate has reacted: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The mean titre is 20.00 cm³. Relative atomic masses: C = 12.0, O = 16.0, Na = 23.0.

   (a)

   What is the concentration of the sodium carbonate solution?

   [1 mark]

   • A0.0125 mol dm⁻³
   • B5.30 mol dm⁻³
   • C0.500 mol dm⁻³
   • D0.0500 mol dm⁻³

   (b)

   Which indicator and colour change are suitable for the end-point of this titration?

   [1 mark]

   • APhenolphthalein, pink to colourless
   • BMethyl orange, yellow to orange
   • CMethyl orange, red to yellow
   • DPhenolphthalein, colourless to pink

   (c)

   Calculate the concentration of the hydrochloric acid in mol dm⁻³.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   A student heats 0.840 g of sodium hydrogencarbonate: 2NaHCO₃(s) → Na₂CO₃(s) + H₂O(l) + CO₂(g). The carbon dioxide is collected in a gas syringe. The molar volume of a gas under the conditions used is 24.0 dm³ mol⁻¹. Relative atomic masses: H = 1.0, C = 12.0, O = 16.0, Na = 23.0.

   (a)

   Calculate the volume of carbon dioxide, in cm³, produced if all the sodium hydrogencarbonate decomposes.

   [3 marks]

   (b)

   The student collects only 108 cm³ of gas. Calculate the percentage yield of carbon dioxide and suggest two reasons why the yield is below 100%.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   A student makes hydrated copper(II) sulfate crystals, CuSO₄·5H₂O. 2.00 g of copper(II) oxide, CuO, is added in small portions to warm dilute sulfuric acid until no more dissolves. The excess solid is filtered off, the filtrate is heated to concentrate it and then left to crystallise. The crystals are filtered, washed and dried between filter papers. The mass of crystals is 4.80 g. The balance reads to ±0.005 g and the CuO is weighed by difference, using two readings. Relative atomic masses: H = 1.0, O = 16.0, S = 32.1, Cu = 63.5.

   (a)

   Calculate the percentage yield of crystals. Calculate the percentage uncertainty in the mass of copper(II) oxide weighed and comment on whether it explains the yield.

   [6 marks]

   (b)

   Evaluate the procedure by describing three hazards, with a suitable precaution for each, and by explaining one reason why the yield is below 100%, the effect of incompletely drying the crystals, and one way to reduce the percentage uncertainty in the mass of copper(II) oxide.

   [6 marks]

   Total for question 8: 12 marks

End of questions