Question

A hydrocarbon contains 10.5 g of carbon per gram of hydrogen. If 1 L of the vapour of the hydrocarbon at $127{}^{\circ }C$ at 1 atm pressure weights 2.8 g, then the molecular formula mass of the hydrocarbon is ________.

Answer 1

As we know,

$PV=nRT=\frac{w}{M}RT$

or $1=\frac{2.8}{M}\times 0.0821\times 400$

or $M=91.95\approx 92$

Let the formula be $C_a{H}_b$.

So $12a+b=92$

$\frac{12a}{b}=10.5\left(given\right)$

$11.5b=92$

or $b=8$

$\therefore$ $a=7$

Therefore, the formula is $C_7{H}_8$.

molar mass = 12*7+1*8 = 92gm/mole