. In an organic compound of molar mass greater

than 100 containing only C, H and N , the percentage of C is 6 times the percentage of

H while the sum of the percentages of C and H is 1.5 times the percentage of N . What is the least molar mass?

A: 175

B: 140

C: 105

D: 210

Best Answer


Let the compound be CxHyNz.

Mass of C=12x

Mass of H=y

Mass of N=14z

Total Mass =12x+y+14z

Mass % of C= 13x /(12x+y+14z) ×100

(I) Mass % of C=6× mass % of H



(II) Mass % of C + Mass % of H =1.5 (mass % of N)




and y=3z.

So, the compound can be expressed as  or C1.5zH3zNz or C3zH6zN2z.

Now choosing,z=1 , we have C3H6N2.

Molecular weight =3×12+6×1+2×14=70 g/mol

Given molecular weight >100.

So, when we choose z=2, we have C6H12N4.

Molecular weight  which is greater than =6×12+12×1+4×14=140 g/mol which is greater then 100.

Final answer:

The correct option is B: 140 

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