Question

Assertion :Helium shows only positive deviations from ideal behaviour. Reason: Helium is an inert gas.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is B Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion

A gas which obeys the gas laws and the gas equation $PV=nRT$ strictly at all temperatures and pressures are said to be an ideal gas. Deviations from ideal behaviour are observed particularly at high pressures or low temperatures. The deviation from ideal behaviour is expressed by introducing a factor $Z$ known as compressibility factor in the ideal gas equation:

$Z$ may be expressed as$Z=\frac{PV}{nRT}$

Thus in case of real gases $Z$ can be $<1$ or $>1$

For real gases:

$(P+\frac{a{n}^2}{V^2})(V-nb)=nRT$

In the case of Helium attraction between the molecules is negligible because of its very small mass. So $\frac{a}{V^2}$ can be neglected.The equation becomes:

$P(V-b)=RT$

$\frac{PV}{RT}=Z=1+\frac{Pb}{RT}$

$\Rightarrow Z>1$

Hence, helium is an ideal gas that shows only positive deviations from ideal behaviour.