Relative volatility: It is a ratio of volatility of component A to volatility of component B.

Example 6.3: Calculate the relative volatility of an ideal binary mixture of benzene and toluene at 700 mm Hg and at a temperature of 90 °C.

Solution: At 90 °C, vapor pressure of benzene and toluene can be found to be 1.02´10³ mm Hg and 225.1 mm Hg, respectively. Volatilities of these components can be found to be 1.34, and 0.296. Relative volatility can be found by taking a ratio of volatility of more volatile component and volatility of less volatile component. The results are summarized below.

t/°C	P1	P2	K1	K2	α
90	1.02´10^3	225.1	1.34	0.296	4.53