An alkane has a molecular mass of 86.What is the molecular formula of the alkane? Write all the possible isomers for this molecular formula?

let the formula of the alkane be C_nH_2n+2

Therefore  we have 

12n + 1(2n+2) = 86

12n +2n+2=86

14n = 84

n=6

i.e the formula of the given alkane is C₆H₁₄ !

following are its possible isomers :

1. n-hexane
2. 2-methyl pentane
3. 3-methyl pentane
4. 2,2 dimethyl butane

. The general formula of alkane is CnH2n+2.
Its molecular mass would be = 12 (n) + 1 (2n + 2) = 14n + 2
Given molecular mass = 86
14n + 2 = 86
?
n=6
? Molecular formula of the alkane is C6H14.

The possible isomers for this molecular formula are:

1. CH3 – CH2– CH2–CH2 – CH2 – CH3

2.
CH3 – CH2 – CH2 – CH – CH3
CH3
2 - Methylpentane

3.
CH3 – CH2 – CH – CH2 – CH3
CH3
3 - Methylpentane

CH3
4.
CH3 – CH2 – C – CH3
CH3
2,2 - Dimethylbutane

5.
CH3 – CH – CH – CH3
CH3
CH3
2,3 - Dimethylbutane

Thus, five isomers are possible for C6H14

Let the molecular formula for given alkane isCnH2n+2
so, Given that 12*n+ 1*(2n+2) = 86
n = 6

So alkane will be C6H14

and possible isomers are Hexane and 2-methylpentane.

Thank You
Ruchi
Askiitians Faculty