Question

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?

Answer 1

`"% 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.

`color(blue)("% composition" = "mass of element"/"mass of compound" xx 100)`

You know that your compound is made up of two elements, magnesium, `"Mg"`, and oxygen, `"O"`.

Moreover, you know that magnesium contributes `"2.5 g"` and oxygen contributes `"1.1 g"` to the mass of the compound. This means that you will have

`m_"compound" = m_(Mg) + m_(O)`

`m_"compound" = "2.5 g" + "1.1 g" = "3.6 g"`

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

`"% Mg" = (2.5 color(red)(cancel(color(black)("g"))) )/(3.6color(red)(cancel(color(black)("g")))) xx 100 = "69.4%"`

`"% 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 `"100 g"` of this compound will contain `"69.4 g"` of magnesium and `"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%)`