Here's how you could do that.
- the molar mass of the compound
- the molar mass of the element
The idea here is that in order to find the percent composition of a compound, you need to know exactly how many atoms of each element are present in its molecule or formula unit.
The compound's percent composition is then calculated by taking the total contribution of an element to the mass of one mole of the compound.
In your case, you want to know the percent composition of oxygen in beryllium chlorate,
Start by writing down the molar mass of the compound and that of the element
#"Be"("ClO"_3)_2: " " "175.9146 g/mol"#
#"O: " "15.9994 g/mol"#
Your goal now is to figure out how many moles of oxygen you get per mole of beryllium chlorate. The compound contains beryllium cations,
As you can see from the chemical formula, you get two moles of chlorate anions for every mole of beryllium chlorate. Since each chlorate anion contains three oxygen atoms, you can say that you get
#"1 mole Be"("ClO"_3)_2 -> "6 moles O"#
So, if one mole of beryllium chlorate has a mass of
#(6 xx 15.9994 color(red)(cancel(color(black)("g/mol"))))/(175.9146color(red)(cancel(color(black)("g/mol")))) xx 100 = color(green)("54.57%")#