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A compound has the formula Mg3X2. If 1.87 g of X combines with 4.86 g of magnesium, what is the atomic weight of X?

A) 7.0
B) 14.0
C) 21.0
D) 28.0
E) 32.0

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I'm having a really rough time with this one. It seems the total weight is 6.73 g, and that 27.79% of the compound is X2 and 72.71% is Mg3. 4.86 g is also 0.20 mol of Mg3 I think (4.86 g / 24.31 g/mol = 0.20 mol) (or is it 4.86 g / 3 = 1.62 g, then 1.62 g / 24.31 g/mol = 0.067 mol ? Not sure.). Beyond that, I'm really lost as to what to do.

Thanks to anyone who can help.

3 answers

I think you are on the verge but just need a push. Try this. Isn't 1.87 g X2 = 0.0667 mol X2? Isn't 6.73 g (0.0667 mol) Mg3X2? Check my thinking.
I wrote 3Mg + X2 ==> Mg3X2.
So if we have 0.2 mol Mg, then 1/3 of that is 0.0667 mol X2 as well as 1/3 of that for mol Mg3X2.

thanks i got b!!!!!

I did, too. Could that be N2?