Modern pennies are composed of zinc coated with copper. A student determines the

Question

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.481g and then makes several scratches in the copper coating (to expose the underlying zinc). The student puts the scratched penny in hydrochloric acid, where the following reaction occurs between the zinc and the HCl (the copper remains undissolved):
Zn(s)+2HCl(aq)?H2(g)+ZnCl2(aq)
The student collects the hydrogen produced over water at 25 ?C. The collected gas occupies a volume of 0.896L at a total pressure of 791mmHg .

Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)

Explanation / Answer

Step1 we have to convert volume under given conditions to STP

P1= 792/760; V1=.896l; T1=298K

P2=1atm ; V2=? ; T2=273K

V2 = [(791/760 )x (.896/298 ) x(273/1)=0.854l
moles of H2 = V2/22.4=0.038moles Moles of Zn = Moles of H2=0.038moles Mass of Zn = 65 x Moles of Zn=2.47 percentage of Zn = [Mass of Zn/2.481 ]x100=99.9%

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