The mole and molar massEdexcel A-Level Chemistry: Mind map

Exam questions on The mole and molar mass

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.
   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
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 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
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.
   Calculate the amount (mol) of anhydrous magnesium sulfate and of water driven off, and hence find the value of x.3 marks