Relative Molecular Mass

Relative molecular mass is the ratio of the mass of one molecule of a substance to the mass th 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

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

Kenyata macdona said...

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

Ridwan Abrar said...

Thank you

Ridwan Abrar said...

Thank u

SANDIP SHROFF said...

C4H4
Butene

SANDIP SHROFF said...

Butene
C4H4

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

Bijay Kanojiya said...

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

Bijay Kanojiya said...

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

Rohit Baidya said...

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

Butene is C4H8

anonymous said...

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

aanya said...

Butene c4h8 molecular formula
Ch3chchch3,ch2chch2ch3,cyclobutane,methylcyclopropane

victor Amaechi said...

Nnah ehh theses things are confusing me please where do a learner start from

this is B said...

Useful , Thanks

Something strange is happening here:
Vapour density is measured in kg/m3
Relative molecular mass is dimensionless.

So wouldn't it be strange that Relative molecular mass = 2* Vapour density ??

Unknown said...

Me too, found it quite vexing before accessing this webpage.

Sonu said...

Vapour density too means relative density

Unknown said...

A mixture of methane and oxygen has vapour density 10. The % (by mole) of methane is ?? Pls help with this question sir