Vapour Pressure - All volatile liquids spontaneously
evaporate. When they do the vapour produces a
pressure. This pressure depends on temperature. The
higher the temperature, the greater the rate of
evaporation, the greater the vapour pressure.
Vapour pressure depends on the attractive forces
between particles. The greater the force of attraction
the lower the vapour pressure. Alcohol has a greater
vapour pressure than water because the attractive forces
between alcohol molecules are weaker than between
water molecules.
When gases are collected over water
there is a mixture of water vapour and
the gas being collected. Both gases
together have a pressure equal and
opposite to air pressure.
Sample Problem - If Carbon dioxide
gas is collected at 21oC what is the
pressure of the gas alone if the air
pressure is 102.24 kPa?
At 21oC Vapour
pressure of
water is
2.49 kPa
102.24 kPa = 2.49 kPa + PCO2
PCO2 = 102.24 kPa - 2.49 kPa = 99.75 kPa
If hydrogen gas is collected over water at
12oC and air pressure is 98.2 kPa what is
the pressure of the hydrogen gas alone?
If carbon dioxide gas is collected over
water at 17oC and air pressure is 99.2 kPa
what is the pressure of the carbon dioxide
gas alone?