Chapter 2 Atoms, Molecules, and Ions - Study Questions - Page 95e: 81

$66.46\%$ $15.0\ g\ CuS$

Atomic weights (g/mol): $Cu: 63.546,\ S: 32.06$ Molar mass of CuS: $95.61\ g/mol$ Mass percent of Cu: $63.546/95.61\times100\%=66.46\%$ For 10.0 g of Cu: $10.0\ g\ Cu\div 0.6646\ g\ Cu/g\ CuS = 15.0\ g\ CuS$