Physical Properties of Gases
Chapter 21- continued
Pressure and units:
Since pressure is the force exerted on a unit area of a surface, we can write the relationship: Pressure force/area or P F/A
Over the years, scientists in different countries have used different units to measure force and area, so there are a number of
different units of pressure.
There are four common units of gas pressure:
 millimetres of mercury (mmHg)
 atmospheres (atm)
 pascals (Pa or N m 2)
 bars (bar).
Volume
There are also a number of different units used for volume.
Questions: Q5 and Q6
Boyle’s law
As long ago as 1662, Robert Boyle (Figure
21.8), an Irish chemist, showed by
experiment that for a given amount of gas at
constant temperature, the volume of the gas
is inversely proportional to its pressure. In
other words, if the volume of a fixed
amount of gas at constant temperature is
halved, the pressure will double. If the
volume is tripled, the pressure drops to onethird of its original value. Boyle’s law
quantifies the relationship between the
pressure exerted by a gas and the volume it occupies.
where V1 is the volume at pressure P1, and V2 is the new volume at the new pressure P2. K is the
constant, as it is constant it can be left out of the equation.
Questions : 8, 9, 10, 11
Charles’ law
The kinetic molecular theory states that an increase in the temperature of a gas increases the average kinetic energy of the
molecules. The molecules move more rapidly and collide with the walls of the container more frequently and with greater force.
This can cause:
 the volume of gas to increase, if the pressure on the gas is fixed. Gas in a syringe or balloon would behave in this way
 the pressure to increase, if the volume of the gas container is fixed, such as in a stoppered flask or a gas cylinder.
Important: A new temperature scale known as the kelvin scale or absolute temperature scale.
The relationship between temperature on the Celsius scale (t) and temperature on the kelvin scale (T) is given by the equation:
T t 273
The temperature 0 K (273°C) is the lowest temperature theoretically possible. At this temperature, all molecules would have
zero kinetic energy.
Charles Law states that the volume of a fixed amount of gas is directly proportional to the Kelvin temperature, provided that the
pressure remains constant.
Where V is the volume of a fixed amount of gas and T stands for temperature in Kelvin
Questions: 12, 13
Molar Volume of a Gas:
If we take 1 mole of any gas, the volume it occupies will depend on temperature and pressure only. We define this volume as the
molar volume (Vm) of a gas.
It is convenient to know the molar volume of a gas under the conditions that normally exist in a laboratory.
The temperature of 25°C (298 K) and a pressure of 101.3 kPa approximate to laboratory conditions. These conditions are known
as standard laboratory conditions (SLC).
Standard temperature and pressure (STP) refers to a temperature of 0°C (273 K) and a pressure of 101.3 kPa.
It is therefore usual to assume that the molar volume of a gas at:

SLC is 24.5 L mol-1

STP is 22.4 L mol
From these values, we can be calculate the amount of a gas, given its volume at SLC or STP.
Questions: 16 & 17