Question

At ${40}^{\circ }C$ the vapour pressure ( in torr ) of a mixture of methyl alcohol and ethyl alcohol is represented by $P=199x+135$
(where $x$ is the mole fraction of methyl alcohol)

What are the vapour pressures of pure methyl alcohol and pure ethyl alcohol at ${40}^{\circ }C$ ?

• A. $135$ and $254$ torr
• B. $119$ and $135$ tor
• C. $119$ and $254$ torr
• D. $334$ and $135$ torr

Answer 1

Answer: (D)

Step 1:

Given that, $P=199x+135$

where x is the mole fraction of methyl alcohol.

As we know, the formula of total pressure is,

Total pressure, $P_T=x(P_a^o-P_b^o)+P_b^o$

$\therefore P_a^o-P_b^o=199and{P}_b^o=135torr$

Step 2:

To find $P_a^o,P_a^o-135=199$

$P_a^o=334torr$

Therefore, the pressure of methyl alcohol is 334 torr and of ethyl alcohol is 135 torr.

Hence, option (D) is correct.