AQA AS Level Physics Paper 2, November 2021: Question 6

1 mark · Medium difficulty · Multiple Choice

Calculate the specific charge of an oxygen-15 ion formed by an atom gaining two electrons.

Practise this question

Question

Multiple choice question asking for the specific charge of an oxygen-15 atom that gains two electrons. Four options are provided: A, -1.3 x 10^7 C kg^-1; B, -2.4 x 10^7 C kg^-1; C, -5.1 x 10^7 C kg^-1; D, -6.4 x 10^7 C kg^-1.
Question text

06 An atom of oxygen-15 (15 O) gains two electrons to form an ion.

What is the specific charge of the ion?

[1 mark]

A −1.3 × 107 C kg−1

B −2.4 × 107 C kg−1

C −5.1 × 107 C kg−1

D −6.4 × 107 C kg−1

Mark scheme

Show the mark scheme A single table row indicating that question 06 has the correct key A, corresponding to the answer -1.3 x 10^7 C kg^-1.

Question Key Answer

06 A −1.3 × 107 C kg−1

How to answer it

Specific Charge of an Oxygen-15 Ion

Question 6 • Multiple Choice • 1 Mark

What this question tests

This question assesses your understanding of specific charge (defined as the charge-to-mass ratio of a particle or ion). It tests your ability to calculate the total net charge of an ion from its atomic structure, determine its total mass using nucleon numbers and the unified atomic mass unit, and pay strict attention to algebraic signs (positive vs. negative values).

Exam Breakdown: Question 06

✅ Correct Answer

A: -1.3 × 10⁷ C kg⁻¹

The correct option is A as determined by dividing the net charge of the ion by its total mass.

💡 Key Knowledge

  • Specific Charge Formula: Specific Charge = Total Charge / Total Mass ( Q / m ).
  • Net Charge of Ion: Oxygen-15 has 8 protons and gains 2 extra electrons, giving a net charge of -2e.
  • Mass Approximation: The mass of an atom is approximately equal to its nucleon number multiplied by the mass of a nucleon (approx. mass of a proton/neutron, 1.66 × 10⁻²⁷ kg).

📐 Step-by-Step Calculation

  1. Find Total Charge (Q):
    Ion has gained 2 electrons:
    Q = -2 × 1.60 × 10⁻¹⁹ C = -3.20 × 10⁻¹⁹ C
  2. Find Total Mass (m):
    Nucleon number = 15
    m = 15 × 1.66 × 10⁻²⁷ kg = 2.49 × 10⁻²⁶ kg
  3. Calculate Specific Charge:
    (-3.20 × 10⁻¹⁹ C) / (2.49 × 10⁻²⁶ kg) = -1.285 × 10⁷ C kg⁻¹ (rounds to -1.3 × 10⁷ C kg⁻¹)

❌ Common Errors

  • Sign Errors: Forgetting the negative sign because the ion gained electrons (resulting in a negative specific charge).
  • Using Proton Number for Mass: Using 8 instead of the nucleon number 15 for the mass calculation.
  • Ignoring Ionization: Calculating the specific charge for a neutral oxygen-15 atom ( Q = 0 ) rather than the negatively charged ion.

🧠 Exam Technique & Examiner Insights

In multiple-choice questions involving magnitudes and signs, always eliminate options with the wrong sign first. Since electrons were added, the specific charge must be negative, which immediately rules out any positive distractor values.

Mark allocation: 1 mark for selecting option A. Speed and accuracy in handling standard form indices ( × 10ⁿ ) are crucial here.

Topics

Physics · 3.2 Particles and radiation

Question and mark scheme from the AQA AS Level Physics examination, Paper 2, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.