Question

Choose the correct statement regarding compressibility factor, Z

• A. For most of the real gases, Z decreases with P at the lower pressure
• B. For most of the real gases, Z increases with P at the higher pressure
• C. For $H_2$ (g) and $He\left(g\right)$, Z increases with P at all the pressure & at room temperature
• D. All of the above statements are correct

Answer 1

The correct option is D All of the above statements are correct

(a) For 1 mole van der Waals' equation is
$(P+\frac{a}{V^2})(V-b)=RT$
At low pressure
$Z=1-\frac{a}{VRT}(V\propto \frac{1}{P})$
So Z decreases with P
(b) At higher pressure
$Z=1+\frac{Pb}{RT}$
So Z increase with P
(c) For $H_2\left(g\right)$ & He(g) value of 'a' is extremely small so they are difficult to liquefy. Thus
$Z=1+\frac{Pb}{RT}$
Here Z increases with P at all the pressure.