Chemistry, Third edition

GAS LAWS

If the piston is allowed to cool, the reverse effect occurs, and the piston handle

moves inwards so reducing the volume of gas inside.

Boyle’s law

Think of a sample of gas of volume Vcontained in a piston (Fig. 10.7). If we push the

piston handle inwards, the pressure on the gas exceeds atmospheric pressure and the

volume of the gas drops. Boyle’s law states that for a fixed mass of gas at a particular

temperature

The volume (V) of a gas is inversely proportional to the pressure (P) of the gas.

Mathematically, Boyle’s law is expressed as

V

1

P

Figure 10.7 illustrates the case where the applied pressure increases from 1 atm to

2 atm. This halves the gas volume, as predicted by the above equation.

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Boyle’s law

A gas contained in a piston at a pressure of 30.0 kPa occupies a volume of 200.0 cm^3 at

300 K. The pressure of the gas is slowly increased to 90.0 kPa without a change in

temperature. What is the new volume of the gas?

Exercise 10F

Fig. 10.7Boyle’s law.

Avogadro’s law

Avogadro’s law states that at a fixed pressure and temperature

The volume of a gas is proportional to the number of moles (or molecules) of

gas present.

Mathematically we can write

Vn

wherenis the number of moles of gas.

The following statement follows from Avogadro’s law:

Equal volumes of all gases at the same temperature and pressure contain the

same number of molecules.

It also follows that one mole of anygas occupies the same volume at the same

temperature and pressure. This volume is known as the molar volumeof the gas.

Experiments show that the molar volume of an ideal gas at SATP is 24.79 dm^3. The

molar volume of an ideal gas at 1 atm pressure and 20 °C is 24 dm^3 (Fig. 10.8), and it

is this volume that we will use in routine calculations.