Hmm, I seem to remember doing this sort of stuff. I think you have to assume there are 100g of the compound, and then work out the number of moles. Or something like that...

In MgO there are 50g of each element.

Moles = mass / atomic mass.

Moles Mg = 50g / 24

Moles Mg = 2.083...

1:1 ratio between elements, therefore 2.083 moles O

Mass O = moles x atomic mass

Mass O = 2.083 x 16

Mass O = 33.333... g or percent

Therefore percentage mass of MgO is 66.6 percent Mg and 33.3 percent O

Probably. I think that's right... :?

OK, so you got the right idea there, but your first statement messed you up: If you have 50g of Mg, you have to have 50g of O!

This is how I do it. A simple(ish) table:

Moles:SPACERMgSPA:CERO
Moles:SPACER1MSPA:CER1
RMM:sSPACER24SPA:CER16
Mass (ratio):M24SPA:CER16

So you have 24 x 100 % mass Mg (60%)
So you have 224+16

And 16 x 100 % mass O (40%)
And 224+16

so i was partially right with the 60% psrt for Mg i just didnt put the 40% for O. thanks guys

Actually, there's a much simpler way of figuring this out. First you need to figure out the molar mass of the compound itself. For MgO, the molar mass is 40.3044 grams per mole. So in one mole of MgO you'll have one mole of Magnesium and one mole of Oxygen. So now you just take the molar mass of magnesium and divide it by the molar mass of MgO. Multiply that by 100 and you get the mass % of magnesium. (Then just take 100 and subtract the mass % of Mg and you'll have the mass % of O).

(24.3050/40.3044)x100 = 60.3036% Mg

100-60.3036 = 39.6964 % O.

Ummm. Duh! I think I should have read the thread in its entirety before making my original post. hehe. allan_chemist already said everything I did in a concise, easy to follow method. :D