# How to calculate the vapour density of a mixture?

I have a gaseous mixture of $$\ce{H2}$$ and $$\ce{CO2}$$ which contains $$66\%$$ by mass of $$\ce{CO2}$$. I have to figure out the vapour density of the mixture (defined as mass of a certain volume of a substance divided by mass of same volume of hydrogen).

My approach: let's assume $$\pu{100 g}$$ of mixture contains $$\pu{66 g}$$ of $$\ce{CO2}$$ and $$\pu{34 g}$$ of $$\ce{H2}$$. Hence, the amount of substance of $$\ce{CO2} = \pu{1.5 mol}$$ and the amount of substance of $$\ce{H2} = \pu{17 mol}$$. The total amount of substance $$= \pu{18.5 mol}$$.

The mass of $$\pu{1 mol}$$ of mixture is 5.4 g (mass of 18.5 mol is 100 g), and the vapour density $$\text{VD} = 2.7$$.

I got my answer right, but now I am doubtful of my approach. Was it safe to assume that the amount of substance of mixture equals that of the amount of substance of both the gases?

• And where does that value come from? Confusing. – Karl Jul 16 '17 at 14:15
• as VD= M/2 M(= molecular mass – user8167818 Jul 16 '17 at 14:18
• Vapour density is molar mass divided by two? That sounds strange. Shouldn't it have units of gramm per liter? – Karl Jul 16 '17 at 14:25
• @Karl The OP is right (vapour density). I wasn't sure whether it's $2$ or $29$. – andselisk Jul 16 '17 at 14:50
• @andselisk Thanks. I have never heard of this measure before, and hope I will never again. ;-) – Karl Jul 16 '17 at 15:15