Use of Amount of Substance in Relation to Masses of Pure Substances

1. Describe the relationship between the temperature of a gas and its volume.

Answer: The greater the temperature of a gas, the greater its volume.

2. Describe the relationship between the pressure of a gas and its volume.

Answer: The greater the pressure of a gas, the smaller its volume.

3. Write down the volume that 1 mole of any gas occupies at room temperature and pressure.

Answer: 24.0 dm3

4. Pentane gas reacts with oxygen according to the equation C5H12(g) + 8O2(g)  →  5CO2(g) + 6H2O(l)
What volume of carbon dioxide will be produced if 0.25 dm3 of pentane are reacted? Assume that both gases are at the same temperature and pressure.

Answer: 1 mole of C5H12 forms 5 moles of CO2  ;  5 × 0.25 dm3 = 1.25 dm3 of CO2

5. The balanced equation for the thermal decomposition of copper carbonate is CuCO3(s)  →  CuO(s) + CO2(g). The figure below shows the apparatus a student used to investigate the thermal decomposition reaction. 0.50 g of copper carbonate was heated until the thermal decomposition reaction began. The carbon dioxide gas was collected in a gas syringe and the volume measured. The student repeated the experiment three times. Relative formula mass (Mr) CuCO3 = 123.5

a) Calculate the maximum theoretical yield, in dm3 , of carbon dioxide gas that can be produced from 0.50 g of copper carbonate. (Note: The volume of one mole of any gas at room temperature and pressure is 24.0 dm3)

Answer: mol CuCO3 = mass ÷ Mr =  0.50 g ÷ 123.5 = 0.00405 mol ;
mol CO2 = mol CuCO3 because of 1:1 ratio in balanced equation ;
volume CO2 = moles × 24.0 = 0.00405 × 24.0 = 0.0972 dm3

b) The table below shows the student’s results. Calculate the mean volume of CO2 . Give the uncertainty of the mean value.

Answer: Mean volume = 79 cm3 ± 1.5 cm3

c) Use the answers above to calculate the percentage yield for the reaction.

Answer: % yield = 100 × (actual volume ÷ maximum theoretical volume) = 100 × (0.079 ÷ 0.0972) = 81.3 %

d) Suggest a reason why the percentage yield of the reaction was not 100%.

Answer: not all of the copper carbonate thermally decomposed / the copper carbonate cooled down and the reaction stopped / the reaction didn’t go to completion / the reaction was reversible