Question

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 ∘C. The collected gas occupies a volume of 0.894 L at a total pressure of 794 mmHg . Part A Calculate the percent zinc in the penny. (Assume that all the Zn in the penny dissolves.)

Answer 1

Answer:

97.1%

Explanation:

Using the ideal gas equation, the number of moles of hydrogen gas produced can be calculated from information provided about the volume of gas evolved at a given temperature and pressure.

The stoichiometry of the reaction is now used to obtain the number of moles of Zn that will produce a given number of moles of hydrogen from the balanced reaction equation as shown. This gives us the number of moles of zinc reacted hence the mass of zinc in the coin since it is assumed that all the zinc reacts.

This is now used to calculate the mass percentage of Zn as shown.