Commercial brass, an alloy of Zn and Cu, reacts withhydrochloric acid as follows
ID: 76889 • Letter: C
Question
Commercial brass, an alloy of Zn and Cu, reacts withhydrochloric acid as follows: Zn (s) + 2HCl(aq) -----> ZnCl2(aq) +H2(g) (Cu does not react with HCl) When 0.5065 g of a certain brassalloy is reacted with excess HCl, 0.0985 g is eventuallyisolated. a. What is the composition of the brass by mass? b. How could this result be checked without changing the aboveprocedure? Commercial brass, an alloy of Zn and Cu, reacts withhydrochloric acid as follows: Zn (s) + 2HCl(aq) -----> ZnCl2(aq) +H2(g) (Cu does not react with HCl) When 0.5065 g of a certain brassalloy is reacted with excess HCl, 0.0985 g is eventuallyisolated. a. What is the composition of the brass by mass? b. How could this result be checked without changing the aboveprocedure?Explanation / Answer
(A) (I'm assuming when the question says '0.0985 g is eventuallyisolated', that it's referring to the ZnCl2, and notH2 gas) MW(ZnCl2) = 136.315 g mol-1 ; MW(Zn) = 65.409g mol-1 ; MW(Cu) = 63.546 g mol-1 Calculate the number of moles (n) of zinc chloride: n(ZnCl2) = m / MW = 0.0985 g /136.315 g mol-1 = 7.226x10-4 mol Next use the (balanced) chemical equation to calculate the moles ofZn that reacted, which is the number of moles of Zn in thealloy: n(Zn) = n(ZnCl2) =7.226x10-4 mol Now calculate the mass of Zn in the alloy: m(Zn) = n(Zn) * MW = 7.226x10-4mol * 65.409 gmol-1 = 0.0473 g Since the sample is 0.5065 g, this means that, by mass: %Zn = 0.0473 / 0.5065 * 100% = 9.331% %Cu = 100 - 9.331 = 90.67% (B) Not sure about this one. I guess after reacting the alloy with HClyou could filter out the remaining reactant, which would hopefullybe the Cu component, since the other species are either aqueous orgaseous. If you had the appropriate instruments, you could also dosome sort of elemental analysis, like mass spectrometry orsomething. Looking at the melting point of the alloy (or some otherthermal analysis technique) might also help define the fraction ofeach component.