OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 4

1 mark · Medium difficulty · Multiple Choice

Calculate the mass of sodium that reacts with water to produce 960 cm3 of hydrogen gas at RTP based on a given chemical equation.

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Question

Multiple choice question 4. The equation for the reaction is given as 2Na(s) + 2H2O(l) -> 2NaOH(aq) + H2(g). Four options A (0.46 g), B (0.92 g), C (1.84 g), and D (3.68 g) are provided to determine the mass of sodium needed to produce 960 cm3 of hydrogen gas at RTP.
Question text

4 Sodium reacts with water as shown below.

2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)

Which mass of sodium reacts with water to produce 960 cm3 of hydrogen gas at RTP?

A 0.46 g

B 0.92 g

C 1.84 g

D 3.68 g

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 4 is option C, worth 1 mark.

4 C 1

How to answer it

Calculating Mass from Gas Volume at RTP

What this question tests

This multiple-choice question assesses your ability to interconvert between gas volume at room temperature and pressure (RTP) and moles, apply stoichiometric balancing ratios from a chemical equation, and convert moles into mass using molar mass.

Question 4 Analysis

Full Worked Solution & Mark Breakdown

Mark: 1 mark available. Correct option is C.

✅ Correct Answer: C (1.84 g)

By determining the moles of hydrogen gas and applying the 2:1 stoichiometric ratio, you find the required moles of sodium, which then converts directly to 1.84 g.

💡 Key Knowledge

  • Molar volume of any gas at RTP = 24.0 dm³ mol⁻¹ (or 24,000 cm³ mol⁻¹ ).
  • Stoichiometric ratios come directly from balanced equations: 2Na : 1H₂ .
  • Molar mass of sodium ( Na ) = 23.0 g mol⁻¹ .

🧠 Exam Technique

Always convert units before calculating! Changing cm³ to dm³ early prevents catastrophic power-of-ten errors during multiple-choice exams.

❌ Common Errors

  • Ratio inversion: Multiplying the moles of gas by 2 instead of dividing (or vice versa). Always check the balanced equation coefficients!
  • Unit slip: Forgetting to divide cm³ by 1000 when using 24 dm³ mol⁻¹ .

📐 Step-by-Step Calculation Guide

  1. Step 1: Convert the volume of hydrogen gas into moles.
    Volume = 960 cm³ = 960 ÷ 1000 = 0.960 dm³ .
    Moles of H₂ = 0.960 dm³ / 24.0 dm³ mol⁻¹ = 0.0400 mol .
  2. Step 2: Use the molar ratio to find moles of sodium (Na).
    From the equation 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂0(g) , the ratio of Na : H₂ is 2 : 1 .
    Moles of Na needed = 0.0400 mol × 2 = 0.0800 mol .
  3. Step 3: Convert moles of sodium into mass.
    Mass = Moles × Molar Mass ( Ar of Na = 23.0 g mol⁻¹ ).
    Mass = 0.0800 mol × 23.0 g mol⁻¹ = 1.84 g . Matches option C.

Topics

Module 2: Foundations in chemistry · 2.1 Atoms and reactions

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