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%

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Must be negative.substance increases