Question

At room temperature following reaction goes to completion.
$2AB\left(g\right)+B_2\left(g\right)\to 2A{B}_2\left(g\right)$
$A{B}_2$ is solid with negligible vapour pressure below $0^{\circ }C$. At $300K$, the $AB$ in the smaller flask exerts a pressure of $3atm$ and the larger flask exerts a pressure of $1atm$ at $400K$ when they are separated out by a close valve. The gases are mixed by opening the stop cock and after the end of the reaction the flask are cooled to $250K$

• A. $0.156atm$
• B. $0.3125atm$
• C. $0.625atm$
• D. $3.2atm$

Answer 1

Answer: (C)

$0.625atm$

$n_{AB}=\frac{3\times 8.21}{0.0821\times 300}=1$;
$n_{B_2}=\frac{1\times 8.21}{0.0821\times 400}=0.25$
$2AB+B_2\to 2A{B}_2$
Initial moles 1 0.25
After reaction 0.5 0.5
Pressure of $B_2{P}_{AB}\times (8.21\times 8.21)=0.5\times 0.0821\times 250=P_{AB}=0.625atm$.