How do you do percent composition in chemistry?

To calculate percent by mass, you need to determine two things: the mass of just the element, and the molar mass of the whole compound. Then, you take the molar mass of just the element and divide it by the molar mass of the whole compound, and times by 100%!!!

the catch is that the top numbers are atomic masses the bottom numbers are the numbers of protons and electrons in an uncharged atom. they are the number of that atoms as you would find them on the periodic table. further, the numbers on the left are used to determine the missing numbers on the right

so here we go

atomic mass

239 + 1 = y + 134 + 3 combine like terms

240 = y + 137             subtract 137 from both terms

240 - 137 = y

103 = y

atomic number

94 = z +54 + 0

94 = z + 54               subtract 54 from both sides.

40 = z

answer

The mass has escaped into a gas which you can not see so the law stays true.