The % composition of Ca in the compound is calculated as below
% composition of Ca =mass of Ca/mass of CaS x100
mass of Ca=20 grams
calculate the mass of CaS by
first write the reacting equation
Ca+ S = CaS
then find the moles of each element
moles=mass/molar mass
moles for Ca= 20g/ 40 g/mol = 0.5 moles
moles for S= 16g/32g/mol = 0.5 moles
since Ca and S has the same moles and the reacting ratio between Ca :CaS the moles of CaS is also = 0.5 moles
mass of CaS= moles of CaS x molar mass CaS
=0.5 x72= 36 grams of Cas
the % composition is therefore = 20g/36g x100 = 55.6%