Problem: Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.483 g 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 oC. The collected gas occupies a volume of 0.899 L at a total pressure of 791 mm Hg .Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)

FREE Expert Solution

Dalton’s Law:
at 25°C, water has a vapor pressure of 23.78 mmHg


Ptotal=PGas 1+PGas 2+PGas 3...Ptotal=PH2+Pwater791 mm Hg=PH2+23.78 mm Hg

PH2=767.22 mmHg


Calculate no. of moles of H2 gas:

PVRT=nRTRTn=PVRTn=767.22 mm Hg 1 atm 760 mm Hg0.899 L0.08206L·atmmol·K25°C+273.15 K

n = 0.0371 mol H2


Balanced equation: Zn(s) + 2HCl(aq) → H2(g) + ZnCl2(aq)

  • Mole to mole comparison: 1 mole Zn forms 1 mole H2


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Problem Details

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.483 g 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 oC. The collected gas occupies a volume of 0.899 L at a total pressure of 791 mm Hg .

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

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Our tutors have indicated that to solve this problem you will need to apply the Collecting Gas Over Water concept. If you need more Collecting Gas Over Water practice, you can also practice Collecting Gas Over Water practice problems.