A sample of P was mixed with an excess of oxygen and the mixture ignited - AQA - A-Level Chemistry - Question 10 - 2017 - Paper 2

1. Calculate the amount of carbon dioxide produced:
   The volume of gas before absorption was 335 cm³ and after absorption, it was 155 cm³. Thus, the volume of carbon dioxide absorbed is:
   $V_{CO_2} = 335 \text{ cm}^3 - 155 \text{ cm}^3 = 180 \text{ cm}^3$

2. Convert the volume to m³ for calculation:
   $V_{CO_2} = 180 \text{ cm}^3 = 180 \times 10^{-6} \text{ m}^3$

3. Using the ideal gas equation to find moles of CO₂:
   $n = \frac{PV}{RT}$
   Where:

• $P = 100,000 \text{ Pa}$ (100 kPa)
• $V = 180 \times 10^{-6} \text{ m}^3$
• $R = 8.31 \text{ J mol}^{-1} \text{ K}^{-1}$
• $T = 298 \text{ K}$ (25 °C)

Substituting these values: $n = \frac{100000 \times 180 \times 10^{-6}}{8.31 \times 298} \approx 0.0072 \text{ mol}$

4. Relate moles of carbon dioxide to moles of P:
   Assuming one mole of P yields one mole of CO₂, the moles of P used is also approximately 0.0072 mol.

5. Calculate mass of P used:
   Using the molar mass of P ($C_4H_8O_2$: 88.11 g/mol as an example), the mass in grams is:
   $\text{Mass of P} = n \times \text{Molar mass} = 0.0072 \text{ mol} \times 88.11 \text{ g/mol} \approx 0.635 \text{ g}$

6. Convert mass to mg:
   $0.635 \text{ g} = 635 \text{ mg}$