The zinc in a copper-plated penny will dissolve in...

The zinc in a copper-plated penny will dissolve in hydrochloric acid if the copper coating is filed down in several spots (so that the hydrochloric acid can get to the zinc). The reaction between the acid and the zinc is $2 \mathrm{H}^{+}(a q)+\mathrm{Zn}(s) \longrightarrow \mathrm{H}_{2}(g)+\mathrm{Zn}^{2+}(a q)$. When the zinc in a certain penny dissolves, the total volume of gas collected over water at $25^{\circ} \mathrm{C}$ is 0.951 $\mathrm{L}$ at a total pressure of 748 $\mathrm{mm} \mathrm{Hg}$ . What mass of hydrogen gas is collected?

Key Concepts

Stoichiometry of Chemical Reactions

This concept involves using balanced chemical equations to determine the proportional relationship between reactants and products. In the context of the reaction given (where zinc reacts with acid to form hydrogen gas), stoichiometry is used to relate the number of moles of zinc consumed to the number of moles of hydrogen gas produced. It helps in calculating the theoretical yield of products according to the molar ratios dictated by the balanced equation.

Ideal Gas Law

The Ideal Gas Law (PV = nRT) is crucial for relating the pressure, volume, temperature, and number of moles of a gas. When a gas is collected during a reaction, this law allows one to calculate the number of moles based on the experimental conditions of pressure, volume, and temperature. This step is essential in converting measured physical quantities into a chemically meaningful amount of substance.

Vapor Pressure Correction in Gas Collection

When gases are collected over water, the total measured pressure is a sum of the partial pressure of the gas and the water vapor pressure. Understanding vapor pressure is important because it requires the subtraction of the water vapor pressure at the collection temperature (e.g., 25°C) from the total pressure to obtain the true pressure of the gaseous reactant or product. This corrected pressure is then used in conjunction with the Ideal Gas Law.

Molar Mass and Mass Determination

This concept involves using the molar mass of a substance to convert the number of moles, calculated from the Ideal Gas Law, to a mass. Once the moles of hydrogen gas are determined, the molar mass of hydrogen (approximately 2 g/mol for H?) can be used to determine the total mass of gas produced during the reaction. This conversion is a fundamental step in quantitative analysis in chemistry.

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