How can I find the percent compositions of (NH4)3PO4?

Percentage composition is just a way to describe what proportions of the different there are in a compound.

The elements are the N , H, O, P. They make up the compound.

To find out the percentage composition of a compound. You need: Ar- number bellow the element on , its molecular mass.

Find out the Ar.

And work out you Mr- which is the molecular mass of your whole compound. So i would do it in chunks,

work out the the Mr of NH4, so N + H4: then x that amount by 3 Cause you have 3 NH4's Then + the Ar of P, and 4 O's to your value. This = Mr of your compound.

You then do the Ar of each of your elements (N,H,O,P) over your MR (from above) x by 100 and it gives you the % each element makes up in your compound.

Hope this helps.