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title: "A student performs an experiment to determine the molar mass of an unknown solid metal, \\(\\text{M}\\). A \\(0.0450\\text{ g}\\) sample of \\(\\text{M}\\text{(s)}\\) reacts completely with excess hydrochloric acid according to the reaction below:  \\[\\text{M}\\text{(s)} + 2\\,\\text{H}^+\\text{(aq)} \\rightarrow \\text{M}^{2+}\\text{(aq)} + \\text{H}_2\\text{(g)}\\]  The \\(\\text{H}_2\\text{(g)}\\) produced is collected over water in a gas-measuring tube at \\(35^\\circ\\text{C}\\) and a barometric pressure of \\(755\\text{ torr}\\). The water levels inside and outside the tube are equalized before the gas volume is recorded. In calculating the molar mass of \\(\\text{M}\\), the student uses the barometric pressure (\\(755\\text{ torr}\\)) as the pressure of \\(\\text{H}_2\\text{(g)}\\), failing to account for the vapor pressure of water at \\(35^\\circ\\text{C}\\) (\\(42.2\\text{ torr}\\)). Which of the following best explains how this omission affects the calculated molar mass of \\(\\text{M}\\)?"
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url: "https://nerd-notes.com/ubq/123632/"
date_modified: "2026-09-28T12:01:51+00:00"
---

# A student performs an experiment to determine the molar mass of an unknown solid metal, \(\text{M}\). A \(0.0450\text{ g}\) sample of \(\text{M}\text{(s)}\) reacts completely with excess hydrochloric acid according to the reaction below:

\[\text{M}\text{(s)} + 2\,\text{H}^+\text{(aq)} \rightarrow \text{M}^{2+}\text{(aq)} + \text{H}_2\text{(g)}\]

The \(\text{H}_2\text{(g)}\) produced is collected over water in a gas-measuring tube at \(35^\circ\text{C}\) and a barometric pressure of \(755\text{ torr}\). The water levels inside and outside the tube are equalized before the gas volume is recorded. In calculating the molar mass of \(\text{M}\), the student uses the barometric pressure (\(755\text{ torr}\)) as the pressure of \(\text{H}_2\text{(g)}\), failing to account for the vapor pressure of water at \(35^\circ\text{C}\) (\(42.2\text{ torr}\)). Which of the following best explains how this omission affects the calculated molar mass of \(\text{M}\)?

A student performs an experiment to determine the molar mass of an unknown solid metal, \(\text{M}\). A \(0.0450\text{ g}\) sample of \(\text{M}\text{(s)}\) reacts completely with excess hydrochloric acid according to the reaction below:

\[\text{M}\text{(s)} + 2\,\text{H}^+\text{(aq)} \rightarrow \text{M}^{2+}\text{(aq)} + \text{H}_2\text{(g)}\]

The \(\text{H}_2\text{(g)}\) produced is collected over water in a gas-measuring tube at \(35^\circ\text{C}\) and a barometric pressure of \(755\text{ torr}\). The water levels inside and outside the tube are equalized before the gas volume is recorded. In calculating the molar mass of \(\text{M}\), the student uses the barometric pressure (\(755\text{ torr}\)) as the pressure of \(\text{H}_2\text{(g)}\), failing to account for the vapor pressure of water at \(35^\circ\text{C}\) (\(42.2\text{ torr}\)). Which of the following best explains how this omission affects the calculated molar mass of \(\text{M}\)?

- **A.** The calculated molar mass is too high, because using the total pressure overestimates the moles of \(\text{H}_2\text{(g)}\) produced, which directly increases the calculated mass-to-mole ratio of the metal.
- **B.** The calculated molar mass is too high, because the presence of water vapor occupies space in the tube, causing the calculated partial pressure of \(\text{H}_2\text{(g)}\) to be lower and leading to an underestimation of the moles of \(\text{M}\).
- **C.** The calculated molar mass is too low, because using the total atmospheric pressure overestimates the partial pressure of \(\text{H}_2\text{(g)}\), which leads to an overestimation of the calculated moles of \(\text{M}\) and a smaller mass-to-mole ratio.
- **D.** The calculated molar mass is too low, because neglecting the vapor pressure of water underestimates the number of moles of \(\text{H}_2\text{(g)}\) produced, which decreases the denominator in the molar mass calculation.

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/123632/*