Question

Negative deviation from ideal behaviour is due to fact that :

• A. There exist molecular interaction and PV/nRT>1.
• B. There exist molecular interaction and PV/nRT<1.
• C. Atoms have finite size and PV/nRT>1.
• D. Atoms have finite size and PV/nRT<1.

Answer 1

The correct option is B

There exist molecular interaction and PV/nRT<1.

For positive deviation , PV=nRT+nPb.Thus, the factor nPb is responsible for increasing the PV value above ideal value.The quantity b is actually the effective volume of molecule.So , it is the finite size of molecules that leads to the origin of b and hence positive deviation at high pressure.

Due to appreciable intermolecular forces, the gas is more comprerible in the low pressure region for one mol, the Van der Walls equation is

(P + $\frac{a}{V_m^2}\left)\right(V_m-b)$=RT

At low pressures, $V_m–b\approx V_m$

$\Rightarrow P{V}_m+\frac{a}{V_m}$=RT