15.1 Ideal Gas Law

A cylinder of radius 2.5 cm and height 50 cm is fixed with an airtight piston containing an ideal gas of temperature 15 °C. Before the piston is pushed in the pressure is 2 × 10⁴ Pa and the volume is 4 × 10⁻³ m³. 

The piston is compressed through a height of 20 cm as shown in Fig. 1.1.

Fig. 1.1

Calculate the change in pressure that takes place in the cylinder.

It takes 30 seconds to compress the piston in part (a). The cylinder is made of a material such that the surface area on the inside is the same as the surface area on the outside. Assume that each particle exerts an equal force on the sides of the cylinder. 

When compressed the particles inside the cylinder have a density of 2.3 kg m⁻³. 

A gas syringe is connected through a delivery tube to a conical flask which is immersed in a beaker of boiling water. The water is being heated by a constant bunsen burner flame as shown in Fig. 1.1. The syringe is frictionless so the gas pressure within the system remains equal to the atmospheric pressure 1.02 × 10³ Pa. 

Fig 1.1

The total volume of the conical flask and delivery tube is 325 cm³, and after settling in the boiling water the gas syringe has a volume of 30 cm³.

 Calculate the total number of moles contained within the system.

The bunsen burner is turned off and the whole system is placed in a freezer. The conical flask is left to cool in the beaker of water which turns into ice.  

It takes 5 minutes for all the water to stop boiling and then 3 hours to cool until the point before the water turns to ice. It then takes a further 1.5 hours for all the water to turn into ice around the conical flask.  

Sketch, on Fig. 1.2, a graph to show this process.

The mass of the ice and the water in the beaker is 240 times the mass of the gas in the system. 

• Specific latent heat of vapourisation = 2160 kJ kg⁻¹
• Specific latent heat of fusion = 324 kJ kg⁻¹
• Specific heat capacity of water = 4084 J kg⁻¹ °C⁻¹

Calculate the number of molecules in a gas containing 120 g of . 

A different ideal gas has an initial volume of 6.00 × 10⁴ m³ at a pressure of 1.42 × 10⁵ Pa and a temperature of 8 °C 

It is heated at a constant volume so that, in its final state, the pressure is 1.70 × 10⁵ m³ at a temperature of 63 °C. 

Show that these two states prove it behaves as an ideal gas.

Atoms of a real gas each have a diameter of 0.12 nm. 

Estimate an accurate value for the volume occupied by 0.54 moles of this gas.

volume = ........................... m³

Explain the differences in the volumes of the atoms in part (b) and part (c).

The air in a bicycle tire has a constant volume of 3.5 × 10⁶ mm³. The pressure of 0.29 mol of the air is 2.0 × 10⁵ Pa. The air may be considered to be an ideal gas. 

The pressure of the tire is increased using a pump. On each stroke of the pump, 0.0045 mol of air is forced into the tire. 

Calculate the number of moles of air in the tire when the pressure increases to 2.7 × 10⁵ Pa and the temperature increases by 10 °C.

Calculate the number of strokes of the pump required to increase the pressure to 2.7 × 10⁵ Pa and the temperature by 10 °C.

State the Pressure law of ideal gases.

The pressure exerted by an ideal gas of 8.4 × 10²⁰ molecules in a container of volume 1.6 × 10^–5 m³ is 3.2 × 10⁵ Pa. 

Calculate the temperature of the gas in the container in ºC.

The pressure of the gas is measured at different temperatures whilst the volume of the container and the mass of the gas remain constant. 

Fig 1.1

Draw a graph on the grid in Fig 1.1 to show how the pressure varies with the temperature.

The container described in part (b) has a release valve that allows gas to escape when the pressure exceeds 4.0 × 10⁵ Pa. 

Calculate the number of gas molecules that escape when the temperature of the gas is raised to 520 °C.

State two equations or laws which ideal gases obey.

A car tyre of volume 4.2 × 10^–2 m³ contains air at a pressure of 300 kPa and a temperature of 290 K. The mass of one mole of air is 3.1 × 10^–2 kg. 

Assuming that the air behaves as an ideal gas, calculate: 

            (i)         The amount of moles of air. 

[3]

            (ii)        The mass of the air. 

[1]

            (iii)       The density of the air.

[1]

A bicycle tyre with 0.47 moles of air has a volume of 1.90 × 10^–3 m³ when the temperature is 252 K. 

Calculate the pressure inside the bicycle tyre.

After the bicycle has been ridden, the temperature of the air in the tyre is 301 K. 

Calculate the new pressure in the tyre assuming the volume is unchanged.

Give your answer to an appropriate number of significant figures.