# What mass of oxygen is found in "1.36 g" of "MgO"?

Jul 5, 2017

The Oxygen is $0.54$ g.

#### Explanation:

Magnesium oxide is composed by atoms of Magnesium and Oxygen in equal quantities because the formula unit is MgO.

From the periodic table we read that the mass of one atom of Magnesium is $24.305$ u and the mass of one atom of Oxygen is $15.999$ u.

This means that one formula unit of MgO has a mass of $24.305 + 15.999 = 40.304$ u and the fraction of mass that is of Oxygen is

$\frac{15.999}{40.304} \setminus \approx 0.397$.

Then when we have a certain quantity of MgO we know that the 39.7% of it by mass is Oxygen (and 60.3% by mass is Magnesium) and in our case the mass of the Oxygen will be

$1.36 \setminus \times 0.397 \setminus \approx 0.54$ $\text{g}$.