Question

Calculate the percent composition by mass of oxygen in litharge (gram formula mss=223.2 grams per mole)

Answer 1

Litharge is a yellow oxide of lead with a formula of PbO, which is formed by heating lead metal in air. 
The mass of one mole of PbO is 223.2 grams
1 mole of PbO contains 16 g of oxygen
Therefore; the percentage of oxygen will be;
     (16/223.2) × 100 = 7.168 %

Answer 2

The  %  composition  by mass  of oxygen in litharge ( grams formula mass=223.2 g/mol) is  calculated as  below

%  composition is   percentage  by mass  of  each   element  in a  compound
which  is  calculated  as follows

%composition = molar mass  of element/molar  mass  of  compound  x100

The  molar mass of oxygen = 16 g/mol

therefore the % composition  = 16 g/mol/ 223.2 g/ mol  x100 = 7.17 %  of oxygen