Modern pennies are composed of zinc coated with copper. A...

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be $2.482 \mathrm{~g}$ 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 $\mathrm{HCl}$ (the copper remains undissolved): $$ \mathrm{Zn}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{H}_{2}(g)+\mathrm{ZnCl}_{2}(a q) $$ The student collects the hydrogen produced over water at $25^{\circ} \mathrm{C}$. The collected gas occupies a volume of $0.899 \mathrm{~L}$ at a total pressure of $791 \mathrm{mmHg}$. Calculate the percent zinc in the penny. (Assume that all the $\mathrm{Zn}$ in the penny dissolves.)

Modern pennies are composed of zinc coated with copper. A student determines the mass of a penny to be $2.482 \mathrm{~g}$ 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 $\mathrm{HCl}$ (the copper remains undissolved):
$$
\mathrm{Zn}(s)+2 \mathrm{HCl}(a q) \longrightarrow \mathrm{H}_{2}(g)+\mathrm{ZnCl}_{2}(a q)
$$
The student collects the hydrogen produced over water at $25^{\circ} \mathrm{C}$. The collected gas occupies a volume of $0.899 \mathrm{~L}$ at a total pressure of $791 \mathrm{mmHg}$. Calculate the percent zinc in the penny. (Assume that all the $\mathrm{Zn}$ in the penny dissolves.)

Key Concepts

Ideal Gas Law

This concept involves using the relationship PV = nRT to calculate the number of moles of a gas when its pressure, volume, and temperature are known. It is essential for connecting the macroscopic behavior of gases to the amount of substance present, allowing quantitative analysis of gas evolution in chemical reactions.

Partial Pressure Correction

When a gas is collected over water, the measured total pressure includes contributions from both the gas of interest and the water vapor. Correcting for the water vapor pressure (using known vapor pressure values at the given temperature) is vital to ensure that calculations accurately reflect the pressure—and, hence, the moles—of the target gas.

Reaction Stoichiometry

Stoichiometry refers to the quantitative relationships between reactants and products in a chemical reaction as derived from a balanced chemical equation. Understanding the mole ratios in a reaction allows one to link the amount of a reactant consumed (or product generated) directly to the amount of another substance involved, which is critical in determining the mass or yield of components.

Percent Composition Calculation

Percent composition is the process of determining what fraction of the total mass of a sample is contributed by a specific component. By combining the mass of the component (found through stoichiometric relationships) with the total mass of the sample, the percent composition provides insight into the purity or formulation of the material.

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