Question

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?

Answer 1

`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*g` of unknown compound.

And thus there are `(60.3*g)/(24.31*g*mol^-1)=2.48*mol` `Mg`, and likewise:

`(39.7*g)/(16.00*g*mol^-1)=2.48*mol` `O`.

We divide thru by the lowest molar quantity to give an empirical formula of `MgO`, as we would have expected.