Question

What is the percentage metal by mass in magnesium oxide?

Answer 1

In the normal way:

`%Mg="Mass of magnesium"/"Mass of magnesium + oxygen"xx100%`

Explanation:

Now it is a fact that magnesium oxide has a formula of `MgO`.

And thus, `%Mg=(24*g*mol^-1)/((24+16)*g*mol^-1)xx100%=??%`

Answer 2

In order to find the % of Mg in MgO, calculate it using this formula:

(mass of element you're looking for) `-:` (molar mass of whole compound) `xx` 100

In that case, the element you want to look for is Mg and the whole compound is MgO.

But first, you need to find the molar mass of MgO.
The molar mass is the same thing as atomic mass on the periodic table.
Mg = `24.3 g mol^-1`
O = `16.0 g mol^-1`
MgO = 24.3 + 16.0 = `40.3 g mol^-1`
To find a molar mass of a compound, break up every element involved. Find the mass of each element involved and add them up to get the molar mass of whole compound. You're basically analyzing the MgO compound.

Now you know the molar mass of MgO, you can now proceed to find the molar mass of Mg.
If you look straight into the periodic table, you can identify the molar mass of Mg. `24.3 g mol^-1`, to be exact.

So the percentage of Magnesium is,
`(24.3 g mol^-1)/(40.3 g mol^-1) xx 100`

is 60.3%. (This answer is rounded to 3 s.f.)