The average molar mass of an equilibrium mixture of N2O4 and NO2 is 69 g/mol (molar mass of N2O4 = 92 g/mol). The degree of dissociation of N2O4 is

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The average molar mass of an equilibrium mixture of $N X_{2} O X_{4}$ and $NO X_{2}$ is $69 g/mol$ (molar mass of $N X_{2} O X_{4} = 92 g/mol$ ). The degree of dissociation of $N X_{2} O X_{4}$ is

M_{a vg} = 92/ (1 + α)

, so

α = 92/69 - 1 = 0.333

, i.e. 0.33. Using

(92 - 69) /92

gives 0.25.