Measuring gas volumes is a very tricky thing.  First and foremost a gas has to be completely enclosed as the molecules are free to move.

Fig.1.  Collecting a gas by the displacement of water.

We can use a measuring cylinder As sown in the diagram to measure the gas collected, but how accurate is the reading. There are three factors that affect the volume of a gas.  1. Number of molecules ( mass)  2.  Temperature  3. Pressure.

Fortunately there are three laws to tackle these environmental conditions.

1.     Boyle’s Law :-  The volume of a gas proportionately decreases  with the  increase of pressure, when the temperature remains the same.

Fig 2. Robert Boyle. 1627 – 1691 ( Irish)

Formula:-  P₁ V₁ = P₂ V₂

       

           Fig.2 Graph to illustrate the Boyle’s law.           Fig. 3.  Pressure against the inverse of volume.

1.     Charle’s Law:-

The volume of a fixed mass of a gas is directly proportional to the absolute temperature when the pressure is constant.

 

Formula:-  V₁ / T₁  =  V₂ / T₂

Fig. 4. Jacques Alexandre César Charles 1746 – 1827 (Fr)

_{Fig. 5 Volume of a gas against the Celsius temperature.}

When the volume of a mass of a gas is plotted against the Celsius temperature we get a straight line graph. But this does not come to the origin.

Careful experiments have shown that the linear graph cuts the Y axis at – 273 C˚.  If you start a temperature taking this point as zero that temperature will be directlt proportianal to volume.  This scale is called the Absolute temperature scale or the Kelvin scale. And denoted by capital T.  Formula:-  V₁ / T₁  =  V₂ / T₂

For a fixed quantity of gas in moles, Boyle's Law and Charles' Law can be combined as follows:-

PV T	= k (a constant)

OR

Combined gas equation.

           PV = n RT

P= Pressure in Pascals. ,    V is the volume in M3 ,  n  means the number of moles of gas.    R is the gas constant which is = 8.31