Question

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

Answer 1

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 `"100 g"` of compound.

`color(blue)(|bar(ul(color(white)(a/a)"% element i" = "mass of i"/"mass of compound" xx 100color(white)(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, `"MgO"`, has a molar mass of `"40.3044 g mol"^(-1)`, which means that one mole of magnesium oxide has a mass of `"40.3044 g"`.

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

• one mole of magnesium, `1 xx "Mg"`
• one mole of oxygen, `1 xx "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. `"40.3044 g"`, will contain

`"24.3050 g Mg" " "` and `" " "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 `"100 g"` of magnesium oxide will contain

`"60.304 g Mg" " "` and `" " "39.696 g O"`