OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 2

Calculating Number of Specific Atoms from Mass

OCR AS Level Chemistry • Multiple Choice Question

What this question tests

This question assesses your ability to interconvert between mass, moles, and the number of constituent particles (atoms/molecules) using the Avogadro constant, alongside applying correct chemical stoichiometry (the ratio of atoms within a chemical formula).

Question 2: Step-by-Step Breakdown

Determining the number of oxygen atoms in 88.0 g of CO₂

✅ Correct Answer: C

Option C ( 2.41 × 10²⁴ ) is the correct choice because 88.0 g of carbon dioxide contains 2 moles of molecules, which equates to 4 moles of oxygen atoms. Multiplying 4 by the Avogadro constant gives 2.41 × 10²⁴ atoms.

💡 Key Knowledge

Molar Mass: Calculated using Ar values (C = 12.0, O = 16.0), giving Mr(CO₂) = 44.0 g mol⁻¹ .
Avogadro Constant (N_A): 6.02 × 10²³ mol⁻¹ (number of particles in one mole).
Stoichiometric Ratio: Each individual molecule of CO₂ contains 2 oxygen atoms.

📐 Step-by-Step Calculation

Find moles of CO₂ molecules:
Moles = Mass / Mr = 88.0 / 44.0 = 2.00 mol
Find moles of oxygen atoms:
Since each CO₂ has 2 oxygen atoms, multiply moles of molecules by 2:
2.00 mol × 2 = 4.00 mol of O atoms
Convert moles of oxygen atoms to number of atoms:
Number of atoms = Moles × Avogadro constant
4.00 × (6.022 × 10²³) = 2.4088 × 10²⁴
Round to appropriate significant figures:
The input data (88.0 g) is given to 3 significant figures, so round the final answer to 3 s.f. → 2.41 × 10²⁴ .

❌ Common Errors & Examiner Pitfalls

Forgetting the atom ratio: Many students calculate the number of molecules of CO₂ (which is 1.20 × 10²⁴, Option B) and forget to multiply by 2 for the oxygen atoms.
Using atomic mass incorrectly: Using incorrect relative atomic masses or mixing up the mass of carbon and oxygen.
Power of 10 slips: Calculator entry errors involving standard form indices. Always double-check your exponent value.

🧠 Exam Technique & Examiner Commentary

In multiple-choice questions, examiners deliberately include distractors (wrong options) based on common student mistakes. For instance:

Option B ( 1.20 × 10²⁴ ) traps students who stop after finding the number of CO₂ molecules.
Option D ( 4.82 × 10²⁴ ) often catches students who accidentally multiply by 4 or miscalculate the Mr.

Top tip: Underline what the question is specifically asking for—in this case, oxygen atoms, not total atoms or molecules!

Marks available: [1 mark] — Awarded for selecting option C.