Vapour Density question

[navneet9431]

Homework Statement

The vapour density of a sample of N2O4 gas is 35. What per cent of N2O4 molecules are dissociated inNO2?

Homework Equations

molar mass = 2 * Vapour Density

The Attempt at a Solution

I tried to solve this question this way,
Let the initial number of moles of N2O4=a
and, assume that the degree of dissociation is α
So,the reaction would be

N2O4 ---------> 2NO2
a ---------------0 (Before Dissociation)
a(1-α)------------2aα (After-dissociation)

And also since VD of N2O4 gas is 35. So,the mass of N2O4 gas would be=35*2=70.
But i do not know what i shall do next?
I will be thankful for any help!

 

[Borek]

molar mass = 2 * Vapour Density

Hint: observed molar mass is a weighted average of the molar masses of components.