If 15.0 grams of ca combines completely with 12.0 grams of s to form a compound, what is the percent composition of s in the compound?

44.4% by weight

Percent composition is the percent by mass in a composition. Since this problem states "combines completely", I will assume that both reactants are fully consumed and that the resulting product has the combined mass of both reactants. If that's the case, then we simply divide the mass of sulfur by the mass of the compound. So: X = 12.0 / (12.0 + 15.0) = 12.0/27.0 = 0.444444 = 44.4%

The percent composition of s in the compound is 44.4%.

No. of grams of ca = 15

no. of grams of S = 12

percent composition of s in the compound = 12 / (15 + 12) x 100

= 12/27 x 100

= 0.44 x 100

= 44.4%