Gas volumes and molar volumeEdexcel A-Level Chemistry: Subtopic test

1. 1
   A technician reacts marble chips (calcium carbonate) with an excess of dilute hydrochloric acid and collects the carbon dioxide produced in a large gas syringe. Everything is measured at room temperature and pressure (rtp), where the molar volume of any gas is 24.0 dm³ mol⁻¹.

   (a)

   A 2.50 g sample of calcium carbonate (M = 100.1 g mol⁻¹) is used. What amount, in mol, of calcium carbonate is this?

   [1 mark]

   • A2.50 mol
   • B0.0250 mol
   • C0.250 mol
   • D40.0 mol

   (b)

   The reaction is CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. What volume of carbon dioxide at rtp is produced when 0.0250 mol of calcium carbonate reacts completely?

   [1 mark]

   • A300 cm³
   • B0.600 cm³
   • C1200 cm³
   • D600 cm³

   (c)

   Calculate the mass of calcium carbonate (M = 100.1 g mol⁻¹) needed to produce 90.0 cm³ of carbon dioxide at rtp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student studies the complete combustion of gaseous hydrocarbons in a eudiometer (a graduated gas-measuring tube). All gas volumes are measured at the same temperature and pressure, and any water formed condenses to a liquid whose volume can be ignored.

   (a)

   Propane burns completely: C₃H₈(g) + 5O₂(g) → 3CO₂(g) + 4H₂O(l). What volume of oxygen is needed to burn 40 cm³ of propane?

   [1 mark]

   • A200 cm³
   • B40 cm³
   • C120 cm³
   • D240 cm³

   (b)

   Which statement best explains why, for gases measured at the same temperature and pressure, volumes are in the same ratio as the coefficients in a balanced equation?

   [1 mark]

   • AEqual volumes of different gases have equal masses
   • BAll gas molecules are the same size
   • CEqual volumes of gases contain equal numbers of molecules
   • DAll gases have the same relative molecular mass

   (c)

   25 cm³ of ethene, C₂H₄, is burned in 100 cm³ of oxygen. The equation is C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l). Calculate the total volume of gas remaining when the mixture returns to the original temperature and pressure.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Car airbags are inflated by the rapid decomposition of sodium azide, NaN₃, in a gas generator: 2NaN₃(s) → 2Na(s) + 3N₂(g). Take the molar volume of gas as 24.0 dm³ mol⁻¹ at room temperature and pressure and the relative formula mass of NaN₃ as 65.0.

   (a)

   An airbag must be inflated with 60.0 dm³ of nitrogen at rtp. Calculate the mass of sodium azide needed.

   [3 marks]

   (b)

   A different generator contains 50.0 g of sodium azide. Calculate the volume of nitrogen it produces at rtp and deduce whether this is enough to fill a 30.0 dm³ airbag completely.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A class carries out Core Practical 1 to measure the molar volume of hydrogen. Each student reacts a weighed piece of magnesium ribbon with excess dilute sulfuric acid in a conical flask connected to a 250 cm³ gas syringe, at room temperature and pressure. The accepted molar volume under the laboratory conditions is 24.0 dm³ mol⁻¹.

   (a)

   Describe how the molar volume of hydrogen can be measured in this practical, including how the result is calculated.

   [6 marks]

   (b)

   One student used 0.115 g of magnesium and collected 105 cm³ of hydrogen. The balance has an uncertainty of ±0.001 g and the gas syringe ±0.5 cm³. Calculate the student's value of the molar volume and evaluate whether measurement uncertainty alone can explain its difference from 24.0 dm³ mol⁻¹. Suggest one improvement.

   [6 marks]

   Total for question 4: 12 marks

End of questions