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(A) The degree of dissociation $(\alpha)$ can be calculated from vapour density data using the formula $\alpha = \frac{D-d}{(n-1)d}$,where $D$ is the

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$I$ and $III$

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(A) The degree of dissociation $(\alpha)$ can be calculated from vapour density data using the formula $\alpha = \frac{D-d}{(n-1)d}$,where $D$ is the theoretical vapour density,$d$ is the observed vapour density,and $n$ is the number of moles of products formed from $1$ mole of reactant.This method is applicable only when there is a change in the number of moles during the reaction,i.e.,$\Delta n_g 
eq 0$.Let us calculate $\Delta n_g$ for each reaction:$I. \ 2HI_{(g)} ightleftharpoons H_{2(g)} + I_{2(g)} \Rightarrow \Delta n_g = (1+1) - 2 = 0$$II. \ 2NH_{3(g)} ightleftharpoons N_{2(g)} + 3H_{2(g)} \Rightarrow \Delta n_g = (1+3) - 2 = 2$$III. \ 2NO_{(g)} ightleftharpoons N_{2(g)} + O_{2(g)} \Rightarrow \Delta n_g = (1+1) - 2 = 0$$IV. \ PCl_{5(g)} ightleftharpoons PCl_{3(g)} + Cl_{2(g)} \Rightarrow \Delta n_g = (1+1) - 1 = 1$Since $\Delta n_g = 0$ for reactions $I$ and $III$,the degree of dissociation cannot be calculated using the vapour density method for these reactions.

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