GAS LAWS
Boyle’s Law
Boyle’s law deals with the relationship between pressure and volume of a fixed mass of a gas when
temperature is kept constant.
Pressure in a gas is as a result of the collisions of the gas molecules with the walls of the container. When
the volume of the fixed mass of a gas is decreased through compression at constant temperature, the
molecules travel a shorter distance to collide with the walls of the container, leading to increased number of
collisions per unit time. The pressure of the gas therefore increases with the increased rate of collisions.
Boyle’s law states that the volume of a given mass of a gas is inversely proportional to its pressure at constant
temperature.
What Boyle’s law implies is that as the pressure increases, the volume decreases. The pressure of the gas
inside the barrel of a pump is directly proportional to the physical pressure applied to compress the gas. A
graph of the physical volume is a curve as shown below:.
The mathematical expression of Boyle’s law is:
Hence, VP = Constant
The expression implies that when the volume of a fixed mass of a gas changes from V1 to V2 its pressure
also changes from P1 to P2. This leads to the general expression:
P1V1 = P2V2
When a graph of pressure of a fixed mass of gas is plotted against the reciprocal of volume, a straight line is
obtained.
, -2- PHYSICAL CHEMISTRY
The SI unit of pressure is the Pascal (Pa). It is equal to one Newton per square metre (NM-2). Other units used
to express pressure are atmospheres. One atmospheric pressure is equal to 760 mmHg pressure or 1.01325
× 105 Pascals. The SI unit of volume is cubic metres (m3). One cubic metre is equal to 1.0 × 106 cubic
centimetres (cm3).
Worked Examples
1. A volume of 375 cm3 of a gas has a pressure of 20 atmospheres. What will be its volume if pressure is
reduced to 15 atmospheres?
Solution
From Boyle’s law, P1 V1 = P2V2
P1 = 20 atmospheres, P2 = 15 atmospheres, V1 = 375 cm3, V2 = ?
Substituting for P1, V1 and P2 the equation becomes;
2. A given mass of gas occupies a volume of 200 cm3 at a pressure of 5 atmospheres. At what pressure
will the gas have a volume of 800 cm3?
Solution
From Boyle’s law, P1V1 = P2V2
P1 = 5 atmospheres, P2 = ?, V1 = 200 cm3, V2 = 800 cm3
5 × 200 = P2 × 800
P2 = 1.25 atmospheres
3. A certain mass of gas occupies 250 cm3 at 25°C and 750 mmHg. Calculate its volume at 25°C if pressure
changes to 760 mmHg in SI Units.
Solution
Boyle’s Law
Boyle’s law deals with the relationship between pressure and volume of a fixed mass of a gas when
temperature is kept constant.
Pressure in a gas is as a result of the collisions of the gas molecules with the walls of the container. When
the volume of the fixed mass of a gas is decreased through compression at constant temperature, the
molecules travel a shorter distance to collide with the walls of the container, leading to increased number of
collisions per unit time. The pressure of the gas therefore increases with the increased rate of collisions.
Boyle’s law states that the volume of a given mass of a gas is inversely proportional to its pressure at constant
temperature.
What Boyle’s law implies is that as the pressure increases, the volume decreases. The pressure of the gas
inside the barrel of a pump is directly proportional to the physical pressure applied to compress the gas. A
graph of the physical volume is a curve as shown below:.
The mathematical expression of Boyle’s law is:
Hence, VP = Constant
The expression implies that when the volume of a fixed mass of a gas changes from V1 to V2 its pressure
also changes from P1 to P2. This leads to the general expression:
P1V1 = P2V2
When a graph of pressure of a fixed mass of gas is plotted against the reciprocal of volume, a straight line is
obtained.
, -2- PHYSICAL CHEMISTRY
The SI unit of pressure is the Pascal (Pa). It is equal to one Newton per square metre (NM-2). Other units used
to express pressure are atmospheres. One atmospheric pressure is equal to 760 mmHg pressure or 1.01325
× 105 Pascals. The SI unit of volume is cubic metres (m3). One cubic metre is equal to 1.0 × 106 cubic
centimetres (cm3).
Worked Examples
1. A volume of 375 cm3 of a gas has a pressure of 20 atmospheres. What will be its volume if pressure is
reduced to 15 atmospheres?
Solution
From Boyle’s law, P1 V1 = P2V2
P1 = 20 atmospheres, P2 = 15 atmospheres, V1 = 375 cm3, V2 = ?
Substituting for P1, V1 and P2 the equation becomes;
2. A given mass of gas occupies a volume of 200 cm3 at a pressure of 5 atmospheres. At what pressure
will the gas have a volume of 800 cm3?
Solution
From Boyle’s law, P1V1 = P2V2
P1 = 5 atmospheres, P2 = ?, V1 = 200 cm3, V2 = 800 cm3
5 × 200 = P2 × 800
P2 = 1.25 atmospheres
3. A certain mass of gas occupies 250 cm3 at 25°C and 750 mmHg. Calculate its volume at 25°C if pressure
changes to 760 mmHg in SI Units.
Solution
