If 20.0 grams of Ca combines completely with 16.0 grams of S to form a compound l, what is the percent composition of Ca in the compound?

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%