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For real gases the relation between p, V and T is given by van der Waals equation:
\left ( p+\frac{an^{2}}{v^{2}} \right )(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 measure of magnitude of intermolecular attraction.
(i) Arrange the following gases in the increasing order of ‘b’. Give reason.O_{2}, CO_{2}, H_{2}, He
(ii) Arrange the following gases in the decreasing order of magnitude of ‘a’. Give reason.CH_{4}, O_{2}, H_{2}
 

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(i) We know that volume α size of the molecules. Hence, the increasing order of the value of 'b' will be:

H_{2}<He<O_{2}<CO_{2}

(ii) 'a' is the Van der Waal's constant which represents the magnitude of intermolecular attraction. An \alpha size of the electron cloud. Hence, the greater the size of the electron cloud, the greater will be the dispersion forces and polarizability of the molecule. Hence, the gases in decreasing order of the magnitude of 'a' will be: CH_{4} > O_{2} > H_{2}

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