# In lab you combine 2.5 g of magnesium, Mg, with 1.1 g of oxygen, O. What is the percent composition by mass of each element in the compound you produced?

##### 1 Answer
Feb 15, 2016

"% Mg" = 69%
"% O" = 31%

#### Explanation:

In order to determine the percent composition of an element that makes up a specific compound, you need to know the mass of the element and the total mass of the compound.

$\textcolor{b l u e}{\text{% composition" = "mass of element"/"mass of compound} \times 100}$

You know that your compound is made up of two elements, magnesium, $\text{Mg}$, and oxygen, $\text{O}$.

Moreover, you know that magnesium contributes $\text{2.5 g}$ and oxygen contributes $\text{1.1 g}$ to the mass of the compound. This means that you will have

${m}_{\text{compound}} = {m}_{M g} + {m}_{O}$

${m}_{\text{compound" = "2.5 g" + "1.1 g" = "3.6 g}}$

The percent composition of each element in this compound will thus be

$\text{% Mg" = (2.5 color(red)(cancel(color(black)("g"))) )/(3.6color(red)(cancel(color(black)("g")))) xx 100 = "69.4%}$

$\text{% O" = (1.1 color(red)(cancel(color(black)("g"))))/(3.6color(red)(cancel(color(black)("g")))) xx 100 = "30.6%}$

This tells you that every $\text{100 g}$ of this compound will contain $\text{69.4 g}$ of magnesium and $\text{30.5 g}$ of oxygen.

Since you only have two sig figs for the masses of the two elements, you must round the values to two sig figs

"% Mg" = color(green)(69%)

"% O" = color(green)(31%)