Ncert Exercises & Solutions

For real gases the relation between p, V and T is given by van der Waal's equation

$(p + \frac{{an}^{2}}{V^{2}}) (V - nb) = nRT$

where, 'a' and 'b' are van der Waals' constants, 'nb' is approximately equal to the total volume of the molecules of a gas. 'a' is the messure of magnitude of intermolecular attraction.

1. Arrange the following gases in the increasing order of 'b'. Give reason.

$O_{2}, {CO}_{2}, H_{2}, He$

2. Arrange the following gases is the decreasing order of magnitude of 'a'. Give reason.

${CH}_{4}, O_{2}, H_{2}$

(i) Molar volume occupied by the gas molecules

\propto

size of the molecules and van der Waals' constant 'b' represents molar volume of the gas molecules. Hence, value of 'b' increases in the following order.

H_{2} < He < O_{2} < {CO}_{2}

(ii) van der Waals' constant 'a' is the measure of magnitude of intermolecular attraction. The magnitude of intermolecular attractions increases with increase in size of electron cloud in a molecule. Hence, for the given gases magnitude of 'a' decreases in the following order

{CH}_{4} > O_{2} > H_{2}

Greater the size of electron cloud, greater is the polarisability of the molecule and greater is the dispersion forces or London forces.

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