Question detail
A sample contains two isotopes of an element with mass numbers 12 and 14. If the abundance of the isotope with mass number 12 is 75%, what is the relative atomic mass of the element?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Atomic structure
Question
- A. 13.0
- B. 12.5
- C. 13.5
- D. 14.0
Answer
13.0
Explanation
The correct option is 13.0. Use Percentage Composition: percentage by mass = (Ar contribution / Mr) x 100. The worked result is 13 %. 13.0 is the best answer because it directly supports the AQA A-Level Chemistry objective to report isotope and mass-spectrometry calculations to an appropriate number of significant figures. This reasoning is anchored to Mass number, isotopes and mass spectrometry in Atomic structure, and it separates isotope from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Significant Figures Mistake
Students often report the relative atomic mass of an isotope without considering significant figures, leading to inaccurate results.
To fix this, identify the number of significant figures required based on the data provided. For example, if the isotopic abundance data is given to three significant figures, ensure your final answer is also reported to three significant figures. Use the formula for relative atomic mass: relative atomic mass = (Σ(isotopic mass × abundance)) / Σ(abundance). Substitute the values, perform the calculation, and round the final answer appropriately.
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