9. Modern pennies are composed of zinc coated with copper. A student determines
ID: 919860 • Letter: 9
Question
9. Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be 2.485 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 C. The collected gas occupies a volume of 0.901 L at a total pressure of 790 mmHg . Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)
Explanation / Answer
Zn + 2HCl --> H2 + ZnCl2
Volume of H2 evolved = 0.901 L
Pressure of H2 at 25 c = total pressure - vapor pressure of water.
790- 23.8 = 766.2 mmhg = 1.008 atm
So that,
moles of H2 produced = 1.008*0.901/(0.0821*298) = 0.037 mol
from equation 1 mol Zn = 1 MOL H2
moles of zinc present = 0.037 moles
mass of zinc present = 65*0.037
= 2.405 grams
So, percentage zinc = (2.405/2.485)*100
=96.78%
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