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Sterling silver is a mixture of silver and copper having a mole ration of silver to copper equal to 7.8 what is the mass percent composition of this mixture?

Ok I know the answer to this and I know how its done: 1 mol Cu weighs 63.5 g
7.8 mol Ag weigh 7.8(107.87)= 841g what i don't understand is why is the 7.8 multiplied with grams of Ag and not Cu?

  • CHem -

    mol ratio Ag/Cu = 7.8 which means
    Ag = 7.8*Cu which means
    that the Ag is whatever the Cu is times 7.8 (Ag is the larger component). OR, said another way,
    7.8 mols Ag to 1 mol Cu; then
    7.8(107.87)g Ag to 63.546 g Cu.
    I get %Ag = almost 93% and Cu slightly more than 7%
    I think Sterling silver is supposed to be 92.5% Ag to 7.5% Cu.

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