OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 3

1 mark · Medium difficulty · Multiple Choice

Calculate the number of hydrogen atoms in 2.50 g of pharmacolite, CaHAsO4•2H2O given its Mr is 216.0

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Question

Multiple choice question 3 asks: How many hydrogen atoms are in 2.50 g of pharmacolite, CaHAsO4•2H2O (Mr = 216.0)? Four options are provided: A, 6.97 x 10^21; B, 2.09 x 10^22; C, 2.79 x 10^22; D, 3.48 x 10^22. A box for the answer is shown at the bottom with a mark allocation of [1].
Question text

3 How many hydrogen atoms are in 2.50 g of pharmacolite, CaHAsO4•2H2O

(Mr = 216.0)?

A 6.97 × 1021

B 2.09 × 1022

C 2.79 × 1022

D 3.48 × 1022

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table shows question number 3 with the correct answer D, worth 1 mark, under assessment objective AO2.2.

3 D 1 AO2.2

How to answer it

Calculating the Number of Specific Atoms from Mass

What this question tests

This question assesses your ability to convert mass to moles using molar mass, apply Avogadro's constant to find the total number of formula units, and use chemical stoichiometry (subscripts and water of crystallisation) to determine the total number of specific constituent atoms.

Question 3 (Multiple Choice)

Full Worked Solution & Breakdown

✅ Correct Answer

D (3.48 × 10²²)

Awarded 1 mark for selecting option D. Tested under AO2.2 (Applying knowledge of stoichiometry and Avogadro constant).

💡 Key Knowledge

  • Moles formula: moles = mass / M_r
  • Avogadro's constant (N_A): 6.022 × 10²³ mol⁻¹
  • Formula breakdown: CaHAsO₄·2H₂O contains 5 hydrogen atoms per formula unit (1 inside the acid arsenate ion + 4 inside the 2 water molecules).

🧠 Exam Technique

In multi-step multiple-choice questions, always calculate intermediate answers clearly on your scratchpad. Watch out for stoichiometry traps involving water of crystallisation—students frequently forget to count the H atoms hidden inside the 2H₂O term!

❌ Common Errors

  • Ignoring Water of Crystallisation: Counting only the H written in CaHAsO₄ (giving 1 H instead of 5).
  • Forgetting Avogadro's Constant: Stopping the calculation after finding the number of moles of the compound.
  • Ratio Inversion: Dividing by the Avogadro constant instead of multiplying when scaling up from moles to atoms.

📐 Step-by-Step Calculation Guide

  1. Find the moles of pharmacolite:
    Moles = mass / M_r = 2.50 g / 216.0 g mol⁻¹ = 0.011574 mol
  2. Determine the number of formula units using Avogadro's constant:
    Number of formula units = 0.011574 mol × 6.022 × 10²³ = 6.9704 × 10²¹ units
  3. Analyze the hydrogen atom stoichiometry per formula unit:
    The chemical formula CaHAsO₄·2H₂O contains:
    • 1 H in CaHAsO₄
    • 4 H in 2H₂O (2 × 2)
    • Total = 5 hydrogen atoms per molecule.
  4. Calculate the total number of hydrogen atoms:
    Total H atoms = 6.9704 × 10²¹ × 5 = 3.485 × 10²² , which rounds to 3.48 × 10²² (matching Option D).

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.