---
title: "A water-treatment laboratory analyzes a sample containing an unknown alkaline earth metal, \\(\\text{M}\\), using mass spectrometry. The instrument is adjusted so that all detected ions are \\(\\text{M}^{2+}\\). Thus, each reported \\(m/z\\) value is the isotope’s mass number divided by the magnitude of the ion’s charge. The peak positions and relative abundances are shown below.  | \\(m/z\\) | Relative abundance (%) | |———|————————| | 42.0    | 0.6                    | | 43.0    | 9.9                    | | 43.5    | 7.0                    | | 44.0    | 82.5                   |  Which choice correctly identifies \\(\\text{M}\\) and justifies the identification from the data?"
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url: "https://nerd-notes.com/ubq/119527/"
date_modified: "2026-08-19T12:41:13+00:00"
---

# A water-treatment laboratory analyzes a sample containing an unknown alkaline earth metal, \(\text{M}\), using mass spectrometry. The instrument is adjusted so that all detected ions are \(\text{M}^{2+}\). Thus, each reported \(m/z\) value is the isotope’s mass number divided by the magnitude of the ion’s charge. The peak positions and relative abundances are shown below.

| \(m/z\) | Relative abundance (%) |
|———|————————|
| 42.0    | 0.6                    |
| 43.0    | 9.9                    |
| 43.5    | 7.0                    |
| 44.0    | 82.5                   |

Which choice correctly identifies \(\text{M}\) and justifies the identification from the data?

A water-treatment laboratory analyzes a sample containing an unknown alkaline earth metal, \(\text{M}\), using mass spectrometry. The instrument is adjusted so that all detected ions are \(\text{M}^{2+}\). Thus, each reported \(m/z\) value is the isotope's mass number divided by the magnitude of the ion's charge. The peak positions and relative abundances are shown below.

| \(m/z\) | Relative abundance (%) |
|---------|------------------------|
| 42.0    | 0.6                    |
| 43.0    | 9.9                    |
| 43.5    | 7.0                    |
| 44.0    | 82.5                   |

Which choice correctly identifies \(\text{M}\) and justifies the identification from the data?

- **A.** \(\text{M}\) is \(\text{Ca}\), because the abundance-weighted mean of the \(m/z\) values is approximately \(43.85\), which is the average atomic mass.
- **B.** \(\text{M}\) is \(\text{Sr}\), because the abundance-weighted mean of the \(m/z\) values is approximately \(43.85\), giving an average atomic mass of approximately \(87.7\ \text{u}\).
- **C.** \(\text{M}\) is \(\text{Sr}\), because the unweighted mean of the four \(m/z\) values is \(43.125\), giving an average atomic mass of \(86.25\ \text{u}\).
- **D.** \(\text{M}\) is \(\text{Sr}\), because the most abundant peak has \(m/z=44.0\), so the average atomic mass is exactly \(88.0\ \text{u}\).

*The answer key and step-by-step explanation are available to logged-in users at https://nerd-notes.com/ubq/119527/*