Question

At high pressure, van der Waals' equation becomes:

• A. $PV=RT$
• B. $PV=RT+\frac{a}{V}$
• C. $PV=RT-\frac{a}{V}$
• D. $PV=RT+Pb$

Answer 1

The correct option is D $PV=RT+Pb$

For one mole, van der Waals' equation becomes:
$(P+\frac{a}{V^2})(V-b)=RT$, where $a$, $b$ are van der Waals constants.
At high pressure, $(P+\frac{a}{V^2})=P$ as value of $\frac{a}{V^2}$ << $P$.
$\therefore P(V-b)=RT$
= $PV-Pb=RT$
$\Rightarrow PV=RT+Pb$