OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 1

1 mark · Medium difficulty · Multiple Choice

Calculate the percentage of 11B atoms in a boron sample given its relative atomic mass and constituent isotopes.

Practise this question

Question

Multiple choice question 1 asking for the percentage of 11B atoms in a boron sample containing isotopes 10B and 11B with a relative atomic mass of 10.8. Four options are provided: A 8.0%, B 20%, C 80%, D 92%. A blank answer box is shown at the bottom left with [1] mark indicated on the bottom right.
Question text

1 A sample of boron contains the isotopes 10B and 11B.

The relative atomic mass of the boron sample is 10.8.

What is the percentage of 11B atoms in the sample of boron?

A 8.0%

B 20%

C 80%

D 92%

Your answer [1]

Mark scheme

Show the mark scheme A single-row table showing the mark scheme for question 1. The answer is C, allocating 1 mark under AO2.2 with no additional guidance text.

AO

Question Answer Marks Guidance

element

1 C 1 AO2.2

How to answer it

Calculating Isotopic Abundance

OCR A-Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your understanding of relative atomic mass (Ar) and how it relates to the isotopic composition of an element. You need to apply algebraic manipulation to solve for an unknown percentage abundance when given the relative atomic mass and mass numbers of two isotopes.

Question 1

Determining the Percentage Abundance of ¹¹B

✅ Correct Answer

C (80%)

Awarded 1 mark for selecting option C.

💡 Key Knowledge

  • Relative Atomic Mass (Ar): The weighted mean mass of an atom of an element compared to 1/12th of the mass of an atom of carbon-12.
  • Isotope Abundances: The sum of all percentage abundances in a sample must always equal exactly 100% (or 1 if using fractions).

🧠 Exam Technique

Set up a clear algebraic equation letting your unknown be x . If the percentage of ¹¹B is x , then the percentage of ¹⁰B must be (100 - x) .

❌ Common Errors

  • Answering for the wrong isotope: Finding the value for ¹⁰B (20%) instead of the requested ¹¹B (80%). Always re-read the final question line carefully!
  • Algebra setup slips: Forgetting to divide the final expression by 100 when working with percentages.

📐 Step-by-Step Calculation

Follow these steps to arrive at option C:

  1. Define variables: Let the percentage abundance of ¹¹B be x . Therefore, the percentage abundance of ¹⁰B is (100 - x) .
  2. Set up the Ar formula:
    Ar = [({mass of ¹⁰B} × {abundance of ¹⁰B}) + ({mass of ¹¹B} × {abundance of ¹¹B})] / 100
    10.8 = [10(100 - x) + 11(x)] / 100
  3. Rearrange and solve:
    10.8 × 100 = 1000 - 10x + 11x
    1080 = 1000 + x
    x = 1080 - 1000 = 80%

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.