## Relative Molecular Mass

Relative molecular mass is the ratio of the mass of one molecule of a substance to the mass ^{t}^{h} of a carbon atom, or 1 amu.

### Vapour Density

Vapour density is the ratio of the mass of a volume of a gas, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.

According to Avogadro's law, equal volumes of all gases contain equal number of molecules.

Thus, let the number of molecules in one volume = n,Cancelling 'n' which is common, we get

However, since hydrogen is diatomic.

When we compare the formulae of vapour density with relative molecular molecular mass, they can be represented as

We can therefore substitute the above equation, and arrive at the below formula

Now on cross multiplication, we have

2 x vapour density = Relative Molecular Mass of a gas##### or,

Relative Molecular Mass = 2 x Vapour density

## 12 comments:

Thank You so much Sir.This was very helpful to me in my Mole Concept chapter.

Sir please help

2.32g of a hydrocarbon on ignition gave 7.3g of carbon(iv)oxide and 2.98g of water. If the vapour density of the hydrocarbon is 28. Calculate

I. The empirical formular

2. Molecular formular

3. Structural formular

4 draw the structures of the possible isomers and give the IUPAC name

Thank you

Thank u

C4H4

Butene

Butene

C4H4

If an equimolar mixture of NO and O2 are placed, then what is the vapour pressure of the mixture

Nice and it is too easy method of deduction of the relation of vappour density and molecular....

Nice and it is too easy method of deduction of the relation of vappour density and molecular....

You are right. That helped me too in my mole concept chapter.

Butene is C4H8

I must say, it is a highly relevant and useful article. Thank you sir!

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