## Chemistry and Chemical Reactivity (9th Edition)

Number of moles of Cu: $75.4\ g\times 89.5/100 /63.546\ g/mol=1.062\ mol$ x mol of $Cu_2S$: $x\times159.15\ g/mol+(1.062-2x)\times 95.61\ g/mol=100\ g\times(100-11.0)/100$ $x=0.391\ mol\ Cu_2S$, $62.22\ g$ $(1.062-2x)=0.280\ mol\ CuS$, $26.77\ g$ Mass percent: $Cu_2S: 62.22\%$ $CuS: 26.77\%$