On analysis a certain compound was found to contain iodine and oxygen in the ratio $254\mathit{ g}$ of iodine and $80\mathit{ g}$ of oxygen The atomic mass of iodine is $127$ and that of oxygen is $16$. What is the formula of the compound? 

The molecular mass of iodine$=127\mathit{ g}/\mathit{mol}$
So, the molecule of iodine present in $254\mathit{ g}$ of iodine$=\frac{254g}{127g/\mathit{mol}}$                                                                                               $=> 2\mathit{ mol}$ 

Now, the molecular mass of oxygen $=16\mathit{ g}/\mathit{mol}$ 

So, the molecule of oxygen present in $80\mathit{ g}$ of oxygen$=\frac{80\mathit{ g }}{16\mathit{ g}/\mathit{mol}}$                                                                                                 $=>5$ 

Therefore, the formula will be $I_2{O}_5.$_.