Deduction of relationship between molecular mass and vapour density

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

relationship between molecular mass vapour density

or,

Relative Molecular Mass = 2 x Vapour density

25 comments:

Unknown said...

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

Unknown said...

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

Unknown said...

Thank you

Unknown said...

Thank u

Unknown said...

C4H4
Butene

Unknown said...

Butene
C4H4

Unknown said...

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

Unknown said...

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

Unknown said...

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

Boop said...

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

Unknown said...

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

aanya said...

Butene c4h8 molecular formula
Ch3chchch3,ch2chch2ch3,cyclobutane,methylcyclopropane

Unknown said...

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

Unknown said...

Please where do a learner like starts from? Am so confused please all should help

Karthik B said...

Useful , Thanks

Unknown said...

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...

U r wrong vapour density doesnot hav unit it is ratio of mass upon mass ...so what u were thinking....plz be carefull

Unknown said...

40 ml of mixture of Co and H2 yielded 15 ml of Co2 after explosion with excess O2 The ℅of H2 in the mixture

Unknown said...

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

Jeet sarkar said...

According to Avogadro's hypothesis at same temp.and pressure equal volume of any gas contains equal number of molecule.
(D)=n-molecules of gas/n-molecules of hydrogen
=Weight of 1-molexule of gas/weight of 1-molecule of hydrogen
=Molecular weight of gas (M)/molecular weight of hydrogen.

Unknown said...

Really it was helping and helped me a lot in solving problems related to gases

Sonu said...

Vapour density too means relative density

Ankita said...

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