A hydrocarbon $X$ decolourizes bromine water and its vapour density is $49$ . Identify the hydrocarbon $X$ .

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Solution

Since ' $X$ ' decolourizes bromine water, it is either alkene or alkyne.
The molecular weight of $X = 2 \times$ vapour density $= 2 \times 49 = 98$
If $X$ is alkene:
The molecular weight $= 98$ g/mol
$C_{n} H_{2 n} = 98$
$12 n + 2 n = 98$
$n = \frac{98}{14} = 7$
$n$ is a whole number
Hence, $X$ is $C_{7} H_{14}$ .

If $X$ is alkyne:
The molecular weight $= 98$ g/mol
$C_{n} H_{2 n} = 98$
$12 \times n + 2 n - 2 = 98$
$14 n = 100$
$\Rightarrow$ $n = \frac{100}{14}$
But $n$ is not a whole number.

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If $X$ is alkene:
The molecular weight $= 98$ g/mol $C_{n} H_{2 n} = 98$ $12 n + 2 n = 98$ $n = \frac{98}{14} = 7$ $n$ is a whole number