The mole and molar massEdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician uses a 22.0 g block of dry ice (solid carbon dioxide, CO₂) for a school demonstration. Relative atomic masses: C 12.0, O 16.0. The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹ and the gas constant, R, is 8.31 J K⁻¹ mol⁻¹.

   (a)

   What is the amount, in mol, of carbon dioxide in the block?

   [1 mark]

   • A0.500 mol
   • B2.00 mol
   • C968 mol
   • D0.0500 mol

   (b)

   How many oxygen atoms are there in the block?

   [1 mark]

   • A3.01 × 10²³
   • B9.03 × 10²³
   • C6.02 × 10²³
   • D1.20 × 10²⁴

   (c)

   The block turns completely into gas in a large bag at 298 K and 101 kPa. Calculate the volume of the gas in dm³, assuming it behaves as an ideal gas.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   An organic liquid contains only carbon, hydrogen and oxygen. Analysis shows that it contains 54.5% carbon, 9.1% hydrogen and 36.4% oxygen by mass. Relative atomic masses: H 1.0, C 12.0, O 16.0. The gas constant, R, is 8.31 J K⁻¹ mol⁻¹.

   (a)

   What is the empirical formula of the liquid?

   [1 mark]

   • ACH₂O
   • BC₂H₄O
   • CC₄H₈O₂
   • DC₂H₂O

   (b)

   In a pV = nRT calculation the volume of the vapour is 112 cm³. What is this volume in m³?

   [1 mark]

   • A1.12 × 10⁻⁵ m³
   • B1.12 × 10⁻³ m³
   • C1.12 × 10⁻¹ m³
   • D1.12 × 10⁻⁴ m³

   (c)

   A 0.320 g sample of the liquid is vaporised and occupies 112 cm³ at 373 K and 101 kPa. Calculate its molar mass and deduce its molecular formula.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student heats 2.46 g of hydrated magnesium sulfate, MgSO₄·xH₂O, until it reaches constant mass. The anhydrous magnesium sulfate left has a mass of 1.20 g. M(MgSO₄) = 120.4 g mol⁻¹ and M(H₂O) = 18.0 g mol⁻¹. The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹.

   (a)

   Calculate the amount (mol) of anhydrous magnesium sulfate and of water driven off, and hence find the value of x.

   [3 marks]

   (b)

   Calculate the percentage by mass of water in the hydrated salt and the number of water molecules driven off. A second student carries out the same experiment and obtains x = 6.4. Suggest a likely experimental error and explain how it gives this result.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student identifies an organic liquid containing only carbon, hydrogen and oxygen. Burning 0.920 g of the liquid in excess oxygen gives 1.76 g of carbon dioxide and 1.08 g of water. In a separate experiment, 0.150 g of the liquid is vaporised and occupies 100 cm³ at 373 K and 101 kPa. Relative atomic masses: H 1.0, C 12.0, O 16.0. The gas constant, R, is 8.31 J K⁻¹ mol⁻¹.

   (a)

   Calculate the empirical formula of the liquid from the combustion data.

   [6 marks]

   (b)

   Use the vaporisation data to calculate the molar mass of the liquid, deduce its molecular formula and comment on the validity of using pV = nRT for this sample.

   [6 marks]

   Total for question 4: 12 marks

End of questions