the molecular formula of an unknown hydrocarbon [closed]








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A student analysed a hydrocarbon X and found it to be containing 43g carbon and 7.2g hydrogen.The relative molecular mass of X was found to be 42.What is the molecular formula of X?




$43\ \mathrm g$ of carbon for $7.2\ \mathrm g$ hydrogen.  

Molar mass of carbon is $12\ \mathrm{g/mol}$ and that of hydrogen is $1\ \mathrm{g/mol}$.


Hence there are $\frac{43\ \mathrm g}{12\ \mathrm{g/mol}} = 3.58\ \mathrm{mol} \approx 3.6\ \mathrm{mol}$ of carbon for every $\frac{7.2\ \mathrm g}{1\ \mathrm{g/mol}}=7.2\ \mathrm {mol}$ of hydrogen.


Hence the empirical formula of the compound is $\ce{CH2}$.


Relative molecular mass of $\ce{CH2}$ is $12+2\times1 = 14$. Given relative molecular mass is $42$.


$\displaystyle\frac{42}{14}=3$


Hence the formula of the compound is $\ce{C3H6}$


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