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Given an alloy containing 70 percent of copper and 30 percent nickel, by weight, determine the composition of this alloy in atomic percent. note the atomic weights of copper and nickel are 63.546 and 58.71 grams per mole, respectively.

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Answer is: 68% of copper and 32% of nickel.
if use 100 g of alloy, than:
m(Cu) = 0,7 · 100 g = 70 g.
m(Ni) = 0,3 · 100 g = 30 g.
n(Cu) = m(Cu) ÷ M(Cu)
n(Cu) = 70 g ÷ 63,546 g/mol
n(Cu) = 1,10 mol.
n(Ni) = m(Ni) ÷ M(Ni)
n(Ni) = 30 g ÷ 58,71 g/mol
n(Ni) = 0,51 mol
n(Cu) : n(Ni) = 1,10 mol : 0,51 mol
%(Cu) = 1,1 mol ÷ 1,61 mol = 0,68 = 68 %. 

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