# How can I find the actual percent composition of magnesium oxide?

Jun 28, 2016

Here's how you can do that.

#### Explanation:

A compound's percent composition essentially tells you how many grams of each constituent element you get per $\text{100 g}$ of compound.

$\textcolor{b l u e}{| \overline{\underline{\textcolor{w h i t e}{\frac{a}{a}} \text{% element i" = "mass of i"/"mass of compound} \times 100 \textcolor{w h i t e}{\frac{a}{a}} |}}}$

This means that in order to find a compound's percent composition, you need to pick a sample and use

• the total mass of the sample
• the mass of each constituent element in this sample

Now, magnesium oxide, $\text{MgO}$, has a molar mass of ${\text{40.3044 g mol}}^{- 1}$, which means that one mole of magnesium oxide has a mass of $\text{40.3044 g}$.

This will be your sample of magnesium oxide. Notice that one mole of magnesium oxide contains

• one mole of magnesium, $1 \times \text{Mg}$
• one mole of oxygen, $1 \times \text{O}$

The molar masses of the two elements are

M_("M Mg") = "24.3050 g mol"^(-1)

M_("M O") = "15.9994 g mol"^(-1)

This tells you that one mole of magnesium oxide, i.e. $\text{40.3044 g}$, will contain

$\text{24.3050 g Mg" " }$ and $\text{ " "15.9994 g O}$

Now you have all that you need to figure out the percent composition of the compound. You will have

"% Mg" = (24.3050 color(red)(cancel(color(black)("g"))))/(40.3044color(red)(cancel(color(black)("g")))) xx 100 = color(green)(|bar(ul(color(white)(a/a)color(black)("60.304 %")color(white)(a/a)|)))

"% O" = (15.9994color(red)(cancel(color(black)("g"))))/(40.3044color(red)(cancel(color(black)("g")))) * 100 = color(green)(|bar(ul(color(white)(a/a)color(black)("39.696 %")color(white)(a/a)|)))

You can thus say that $\text{100 g}$ of magnesium oxide will contain

$\text{60.304 g Mg" " }$ and $\text{ " "39.696 g O}$