Q. In a compound $C, H$ and $N$ are present in $9 :1 : 3.5$ by weight. If molecular weight of the compound is $108$, then the molecular formula of the compound is
UP CPMTUP CPMT 2006
Solution:
Element
Element
ratio
Atomic
mass
Relative
no. of
atoms
Simplest
ratio
Carbon
9
12
$\frac{9}{12}=0.75$
3
Hydrogen
1
1
$\frac{1}{1}=1$
4
Nitrogen
3.5
14
$\frac{3.5}{14}=0.25$
1
$\therefore $ Empirical formula of the compound $=C_{3}H_{4}N$
$n (12\times3 +1\times 4+14\times 1)=108$
or, $n (36+4+14)=108$
or, $54n=108$
or, $n=2$
$\therefore $ Molecular formula of the compound $=C_{6}H_{8}N_{2}$
| Element | Element ratio | Atomic mass | Relative no. of atoms | Simplest ratio |
|---|---|---|---|---|
| Carbon | 9 | 12 | $\frac{9}{12}=0.75$ | 3 |
| Hydrogen | 1 | 1 | $\frac{1}{1}=1$ | 4 |
| Nitrogen | 3.5 | 14 | $\frac{3.5}{14}=0.25$ | 1 |