OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 3

1 mark · Easy difficulty · Multiple Choice

Calculate the number of hydrogen atoms in 0.125 mol of C2H5OH

Practise this question

Question

Multiple choice question 3 asking for the number of hydrogen atoms in 0.125 mol of C2H5OH, with four options A: 7.525 x 10^22, B: 4.515 x 10^23, C: 3.7625 x 10^23, and D: 3.612 x 10^24, and a box for the answer.
Question text

3 What is the number of hydrogen atoms in 0.125 mol of C2H5OH?

A 7.525 × 1022

B 4.515 × 1023

C 3.7625 × 1023

D 3.612 × 1024

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 3 is B, worth 1 mark.

3 B 1 AO2.2

How to answer it

Calculating Number of Specific Atoms from Moles

What this question tests

This question assesses your ability to apply the Avogadro constant (Nₐ) alongside molecular formulae to determine the total number of specific constituent atoms within a given molar quantity of a substance. It targets AO2.2 (applying knowledge of stoichiometry to chemical calculations).

Question 3 (Multiple Choice)

Correct Answer: B

✅ Correct Answer

Option B: 4.515 × 10²³

This is the only option that correctly multiplies the total moles of molecules by Avogadro's constant AND accounts for all 6 hydrogen atoms present in every molecule of ethanol ( C₂H₅OH ).

💡 Key Knowledge

  • Formula analysis: Ethanol is written as C₂H₅OH . Counting the hydrogen atoms carefully gives 5 (in the ethyl group) + 1 (in the hydroxyl group) = 6 hydrogen atoms per molecule.
  • Avogadro constant: 1 mole of any entity contains 6.022 × 10²³ entities ( Nₐ ).
  • Proportionality: The number of atoms scales directly with the number of moles of molecules and the atomicity ratio.

🧠 Exam Technique

Multiple-choice questions often include distractor answers calculated by forgetting intermediate steps (like missing the hydrogen in the -OH group or omitting the atom multiplier entirely). Always write down your formula breakdown before punching numbers into your calculator.

❌ Common Errors

  • Forgetting the hydroxyl H: Counting only the H₅ part of the formula and multiplying by 5 instead of 6.
  • Skipping the atom multiplier: Calculating only the total number of molecules of ethanol rather than individual hydrogen atoms.

📐 Step-by-Step Calculation

  1. Identify total hydrogen atoms per molecule: C₂H₅OH contains 5 + 1 = 6 hydrogen atoms.
  2. Calculate total moles of hydrogen atoms: 0.125 mol × 6 = 0.750 mol of H atoms
  3. Convert moles of atoms to actual number of atoms using Avogadro constant: 0.750 mol × 6.022 × 10²³ mol⁻¹ = 4.5165 × 10²³ (using standard calculator value 4.515 × 10²³ depending on the exact constant programmed).
Mark Allocation: [1 mark] awarded for selecting option B.

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.