Question

At ${25}^{\circ }C$, the total pressure of an ideal solution obtained by mixing equimolar A and B is $200\text{torr}$. What is the vapour pressure (in $\text{torr}$) of pure ${}^{\prime }{B}^{\prime }$ at the same temperature (Vapour pressure of pure ${}^{\prime }{A}^{\prime }$ at ${25}^{\circ }C$ is $100\text{torr}$)?

• A. $100torr$
• B. $200torr$
• C. $300torr$
• D. $400torr$

Answer 1

The correct option is C $300torr$

${\chi }_A={\chi }_B=0.5$
$\text{By Raoult's law,}$
$p_A=x_A\times p_A^o$
$p_B=x_B\times p_B^o$
$P_{\text{total}}=x_A\times p_A^0+x_B\times p_B^{0}200=100\times 0.5+p_B^0\times 0.5$
$p_B^0=\frac{150}{0.5}=300torr$