---
title: "A student investigates a pure hydrocarbon, \\(X\\), with a measured molar mass of \\(56.0\\ \\text{g mol}^{-1}\\). Independent evidence limits the identity of \\(X\\) to cyclobutane or methylcyclopropane, which are structural isomers. During a combustion analysis in limited \\(\\text{O}_2\\), all hydrogen atoms from the sample are recovered in \\(\\text{H}_2\\text{O}\\), but some carbon atoms form \\(\\text{CO}\\). No solid carbon is observed.  | Quantity | Measured value | |—|—:| | Mass of \\(X\\) burned | \\(0.560\\ \\text{g}\\) | | Mass of \\(\\text{CO}_2\\) collected | \\(1.320\\ \\text{g}\\) | | Mass of \\(\\text{H}_2\\text{O}\\) collected | \\(0.720\\ \\text{g}\\) |  The gas leaving the collection apparatus gives a positive test for \\(\\text{CO}\\). By incorrectly assuming complete combustion and using only the collected \\(\\text{CO}_2\\) and \\(\\text{H}_2\\text{O}\\), the student reports an empirical formula of \\(\\text{C}_3\\text{H}_8\\) rather than \\(\\text{CH}_2\\). Which statement correctly explains the discrepancy and determines whether a subsequent complete-combustion analysis based only on product masses could distinguish the candidates?"
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url: "https://nerd-notes.com/ubq/120091/"
date_modified: "2026-08-21T08:41:29+00:00"
---

# A student investigates a pure hydrocarbon, \(X\), with a measured molar mass of \(56.0\ \text{g mol}^{-1}\). Independent evidence limits the identity of \(X\) to cyclobutane or methylcyclopropane, which are structural isomers. During a combustion analysis in limited \(\text{O}_2\), all hydrogen atoms from the sample are recovered in \(\text{H}_2\text{O}\), but some carbon atoms form \(\text{CO}\). No solid carbon is observed.

| Quantity | Measured value |
|—|—:|
| Mass of \(X\) burned | \(0.560\ \text{g}\) |
| Mass of \(\text{CO}_2\) collected | \(1.320\ \text{g}\) |
| Mass of \(\text{H}_2\text{O}\) collected | \(0.720\ \text{g}\) |

The gas leaving the collection apparatus gives a positive test for \(\text{CO}\). By incorrectly assuming complete combustion and using only the collected \(\text{CO}_2\) and \(\text{H}_2\text{O}\), the student reports an empirical formula of \(\text{C}_3\text{H}_8\) rather than \(\text{CH}_2\). Which statement correctly explains the discrepancy and determines whether a subsequent complete-combustion analysis based only on product masses could distinguish the candidates?

A student investigates a pure hydrocarbon, \(X\), with a measured molar mass of \(56.0\ \text{g mol}^{-1}\). Independent evidence limits the identity of \(X\) to cyclobutane or methylcyclopropane, which are structural isomers. During a combustion analysis in limited \(\text{O}_2\), all hydrogen atoms from the sample are recovered in \(\text{H}_2\text{O}\), but some carbon atoms form \(\text{CO}\). No solid carbon is observed.

| Quantity | Measured value |
|---|---:|
| Mass of \(X\) burned | \(0.560\ \text{g}\) |
| Mass of \(\text{CO}_2\) collected | \(1.320\ \text{g}\) |
| Mass of \(\text{H}_2\text{O}\) collected | \(0.720\ \text{g}\) |

The gas leaving the collection apparatus gives a positive test for \(\text{CO}\). By incorrectly assuming complete combustion and using only the collected \(\text{CO}_2\) and \(\text{H}_2\text{O}\), the student reports an empirical formula of \(\text{C}_3\text{H}_8\) rather than \(\text{CH}_2\). Which statement correctly explains the discrepancy and determines whether a subsequent complete-combustion analysis based only on product masses could distinguish the candidates?

- **A.** The reported \(\text{C}_3\text{H}_8\) is hydrogen-rich relative to \(\text{CH}_2\), because incomplete combustion causes the collected \(\text{H}_2\text{O}\) to have a larger mass fraction of hydrogen. Complete-combustion analysis could not distinguish the candidates because structural isomers have the same elemental composition.
- **B.** The reported \(\text{C}_3\text{H}_8\) is carbon-rich relative to \(\text{CH}_2\), because ignoring the produced \(\text{CO}\) makes the calculated amount of carbon too small. Complete-combustion analysis could not distinguish the candidates because structural isomers have the same elemental composition.
- **C.** The reported \(\text{C}_3\text{H}_8\) is hydrogen-rich relative to \(\text{CH}_2\), because ignoring the produced \(\text{CO}\) undercounts carbon while the hydrogen amount is still determined correctly from \(\text{H}_2\text{O}\). Complete-combustion analysis could not distinguish the candidates because structural isomers have the same elemental composition.
- **D.** The reported \(\text{C}_3\text{H}_8\) is hydrogen-rich relative to \(\text{CH}_2\), because ignoring the produced \(\text{CO}\) undercounts carbon while the hydrogen amount is still determined correctly from \(\text{H}_2\text{O}\). Complete-combustion analysis could distinguish the candidates because different atom connectivities produce different stoichiometric masses of \(\text{CO}_2\) and \(\text{H}_2\text{O}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/120091/*