What is the percentage composition of #CuCl_2#?

For a given compound, its percent composition is all about the contribution each element, not each atom, has to the mass of one mole of the compound.

In your case, every mole of copper(II) chloride, #"CuCl"_2#, is composed of

• one mole of copper atoms,
• two moles of chlorine atoms

As you know, the molar mass of a substance tells you the mass of one mole of said substance. In this case, copper(II) chloride has a molar mass of #"134.452 g/mol"#.

This means that one mole of copper(II) chloride has a mass of #"134.452 g"#.

Because you know that you get one mole of copper atoms and two moles of chlorine atoms per mole of copper(II) chloride, you can use the molar masses of the two elements to find how many grams of each you'd get in #"134.452 g"# of the compound.

You will have

#"For Cu: " "63.546 g/mol " -># one mole has a mass of #"63.456 g"#

#"For Cl: " "35.453 g/mol " -># one mole has a mass of #"35.452 g"#

The percent composition of copper will thus be - keep in mind that you have one mole of copper atoms in one mole of copper(II) chloride

#(1 xx 63.546 color(red)(cancel(color(black)("g"))))/(134.452color(red)(cancel(color(black)("g")))) xx 100 = "47.263% Cu"#

The percent composition of chlorine will be - keep in mind that you have two moles of chlorine atoms in one mole of copper(II) chloride

#(2 xx 35.453 color(red)(cancel(color(black)("g"))))/(134.452 color(red)(cancel(color(black)("g")))) xx 100 = "52.737% Cl"#

Therefore, you can say that #"100 g"# of copper(II) chloride will contain #"47.263 g"# of copper and #"52.737 g"# of chlorine.