Question

At ${40}^{o}C$, the vapour pressure in torr of methanol and ethanol solution is $P=119x+135$. Here, $x$ is the mole fraction of methanol. Hence, _______________ .

• A. vapour pressure of pure methanol is 119 torr
• B. vapour pressure of pure ethanol is 135 torr
• C. vapour pressure of equimolar mixture of each is 127mm
• D. mixture is completely immiscible

Answer 1

Answer: (B)

vapour pressure of pure ethanol is 135 torr

Given,

The total vapour pressure of the methanol and ethanol mixture:

$P=119x+135torr$, where $x$is the mole fraction of methanol. $...\left(i\right)$

Now if put $x=1$in equation$\left(i\right)$, we can calculate vapour pressure of pure methanol

$\Rightarrow P_{methanol}=119\left(1\right)+135=254torr$

And to calculate the vapour pressure of pure ethanol, we will substitute $x=0$in equation$\left(i\right)$to get,

$\Rightarrow P_{ethanol}=119\left(0\right)+135=135torr$

Hence, the correct answer will be option B.