1. 1
   Three gases are stored in industrial cylinders: helium (He), nitrogen (N₂) and ammonia (NH₃). Engineers use the ideal gas model to estimate how much gas each cylinder holds, and need to know when the model can be trusted.

   (a)

   Which statement is NOT an assumption of the ideal gas model?

   [1 mark]

   • AThe particles have negligible volume.
   • BThere are no intermolecular forces between the particles.
   • CAll collisions between particles are elastic.
   • DAll the particles move at the same speed.

   (b)

   Under which conditions would nitrogen behave most like an ideal gas?

   [1 mark]

   • AHigh temperature and low pressure
   • BLow temperature and high pressure
   • CHigh temperature and high pressure
   • DLow temperature and low pressure

   (c)

   Explain why ammonia deviates more from ideal behaviour than helium under the same conditions.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A weather balloon is filled with 5.00 m³ of helium at ground level, where the pressure is 101 kPa and the temperature is 17 °C. It rises to an altitude where the pressure is 25.0 kPa and the temperature is −43 °C. Assume no helium escapes and the balloon material does not limit its expansion.

   (a)

   For a fixed mass of helium at constant temperature, which plot gives a straight line passing through the origin?

   [1 mark]

   • APressure against volume
   • BPressure against 1/volume
   • CVolume against temperature in °C
   • DPressure × volume against volume

   (b)

   What is the volume of the balloon at the higher altitude?

   [1 mark]

   • A25.5 m³
   • B20.2 m³
   • C16.0 m³
   • D0.982 m³

   (c)

   Before launch, the sealed steel cylinder that supplied the helium warms in sunlight. Explain, in terms of particles, why the pressure inside the cylinder increases.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student finds the molar mass of a volatile liquid by injecting 0.188 g of it into a gas syringe held in an oven at 100 °C. The liquid vaporises completely, and the vapour occupies 98.5 cm³ at a pressure of 102 kPa.

   (a)

   Calculate the molar mass of the liquid, giving your answer to three significant figures.

   [3 marks]

   (b)

   The liquid is propanone, CH₃COCH₃, which boils at 56 °C (M = 58.09 g mol⁻¹). A second student repeats the experiment at 60 °C and obtains 60.4 g mol⁻¹. Explain why the result at 60 °C is less accurate and why it is too high rather than too low.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   The ratio pV/nRT equals exactly 1 for an ideal gas. A chemist measures this ratio for methane (CH₄) and hydrogen (H₂) at 273 K. At 100 kPa both gases give 1.00. At 5 000 kPa methane gives 0.89 and hydrogen gives 1.03. At 10 000 kPa methane gives 0.79 and hydrogen gives 1.06. A methane molecule has 10 electrons and a hydrogen molecule has 2.

   (a)

   Compare and explain how methane and hydrogen deviate from ideal behaviour as the pressure increases.

   [6 marks]

   (b)

   A cylinder of internal volume 50.0 dm³ holds methane at 10 000 kPa and 273 K. Use the ideal gas equation and the data to determine the actual mass of methane in the cylinder, and evaluate the suggestion that the amount could be found simply by using the molar volume of an ideal gas, 22.7 dm³ mol⁻¹.

   [6 marks]

   Total for question 4: 12 marks

End of questions