Question

At $300K$, the vapour pressure of an ideal solution containing $3mole$ of $A$ and $2mole$ of $B$ is $600torr$. At the same temperature, if $1.5mole$ of $A$ and $0.5mole$ of C (non-volatile) are added to this solution the vapour pressure of solution increases by $30torr$. What is the value of $p_B^0$?

• A. $940$
• B. $405$
• C. $90$
• D. None of these

Answer 1

Answer: (C)

$90$

$P=p_A^0{X}_A+p_B^0{X}_B$
$600=p_A^0\left(\frac{3}{3+2}\right)+p_B^0\left(\frac{2}{2+3}\right)$;
$3p_A^0+2p_B^0=3000$
$630=p_A^0\left(\frac{4.5}{4.5+2+0.5}\right)+p_B^0\left(\frac{2}{4.5+2+0.5}\right)$
$4.5p_A^0+2p_B^0=4410$
$1.5p_A^0=1410;p_A^0=940$ and $p_B^0=90$