When a 13.60-g sample of a compound containing only magnesium and oxygen is decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound?

Dec 10, 2016

O: % $=$ (5.40*g)/(13.60*g)xx100%=39.7%

Mg: % $=$ (100-39.7)%=60.3%

Explanation:

All we have done is to divide the elemental mass by the compound mass, and made it into a percentage.

We could also use these percentages to give an empirical formula. We assume that there are $100 \cdot g$ of unknown compound.

And thus there are $\frac{60.3 \cdot g}{24.31 \cdot g \cdot m o {l}^{-} 1} = 2.48 \cdot m o l$ $M g$, and likewise:

$\frac{39.7 \cdot g}{16.00 \cdot g \cdot m o {l}^{-} 1} = 2.48 \cdot m o l$ $O$.

We divide thru by the lowest molar quantity to give an empirical formula of $M g O$, as we would have expected.