Bolye's Law
Robert Boyle, in 1662, recorded the first set of observations between pressure and volume of air. His observations, given below, were also found to be applicable to other gases. At constant temperature, the volume of a definite mass of a gas is inversely proportional to its pressure. Mathematically, we write it as
V Î± 1 / p
orV = K/p or pV = K (5.1)
Where K is constant of proportionality. Its value depends upon the mass and temperature of the gas.
According to eq. (5.1), the product of pressure and volume of a given mass of a gas remains constant provided its temperature is held constant. Such behavior is shown in Fig.
p_{1}V_{1 }= p_{2}V_{2} (5.2)
Where V_{1} and V_{2} are the volumes of the gas at pressures p_{1} and p_{2}, respectively.

A weather balloon has a volume of 175 dm^{3} when filled with hydrogen at pressure of 1 atm. Calculate its volume at the height of 2 km where the atmospheric pressure is 0.8 atm. Assume that the temperature is constant.
Solution
We have
p_{1} = 1 atm p_{2} =0.8 atm
V_{1} = 175 dm^{3} V_{2 }= ?
Making use of Boyle's law (p_{2}V_{2} = P_{1}V_{1}), we get
= 218.75 dm^{3m }