Question

Calculate % composition: Brass is an alloy made of copper and zinc. The density of copper is 8.96 g/mL and the density of zinc is 7.14 g/mL. If the density of a sample of brass is 8.72 g/mL, what is the percent copper in the brass?

Hint: “Grandma’s chocolates” are composed of a caramel center and a chocolate coating. The measured density of a “Grandma’s chocolate” is 1.045 g/cm3. Knowing that the density of her caramel is 1.160 g/cm3 and that of her chocolate coating is 0.960 g/cm3, the percent by mass of caramel in a “Grandma’s chocolate” can be found from the equation given below. d(“Grandma’s chocolates”) = d(caramel)×(% caramel) + d(chocolate)×(1 ‒ % caramel) where d is density in g/mL, and % caramel is the % by mass (expressed as a decimal) of caramel in a “Grandma’s chocolate.” Since these are the only two components, (1 ‒ % caramel) is the % by mass of chocolate in a “Grandma’s chocolate.” So for the values given: 1.045 g/mL = 1.160 g/mL ´(% caramel) + 0.960 g/mL ´(1 ‒ % caramel) Solving gives % caramel = 0.425 (or 42.5%) (and % chocolate = 57.5%) Hint 2: Answer includes 3 significant figures.

Answer 1