OCR A-Level Chemistry AS Breadth in chemistry (01), June 2018: Question 3
1 mark · Medium difficulty · Multiple Choice
Identify the compound with a given percentage composition by mass from four multiple-choice options.
Practise this questionQuestion
Question text
3 A compound has the composition by mass:
H, 5.00%; N, 35.00%; O, 60.00%.
Which compound has this composition?
A HNO3
B NH4NO3
C HNO2
D NH2OH
Your answer [1]
Mark scheme
Show the mark scheme
3 B 1
How to answer it
Determining Compounds from Percentage Composition by Mass
What this question tests
This question assesses your ability to calculate empirical formulae from percentage composition by mass data, or conversely, to verify the percentage composition of given molecular/ionic compounds. It tests fundamental mole calculations, handling atomic masses (Ar), and working with ratios.
Determining the Correct Compound Formula
✅ Correct Answer
B: NH₄NO₃
💡 Key Knowledge
- Percentage Mass Formula: (Number of atoms × Ar / Mr of compound) × 100
- Alternatively, find the empirical formula by dividing percentage mass by Ar to find relative moles, then simplify the ratio.
- Remember that ammonium nitrate contains polyatomic ions: ammonium ( NH₄⁺ ) and nitrate ( NO₃⁻ ).
🧠 Exam Technique
In multiple-choice questions with formulae options, you don't always have to calculate the empirical formula from scratch. You can work backwards by calculating the percentage mass of each option, or test the ratio of elements using moles.
❌ Common Errors
- Forgetting that compounds like NH₄NO₃ contain two different nitrogen environments (an ammonium ion and a nitrate ion), meaning total N must account for all nitrogen atoms in the formula ( N₂H₄O₃ ).
- Using atomic numbers instead of relative atomic masses (Ar) when calculating moles.
📐 Step-by-Step Calculation / Verification Method
Let's check option B (NH₄NO₃), which can be written molecularly as N₂H₄O₃ :
- Find the molar mass (Mr) of NH₄NO₃:
Mr = (2 × 14.0) + (4 × 1.0) + (3 × 16.0) = 28.0 + 4.0 + 48.0 = 80.0 g mol⁻¹ - Calculate percentage mass of Hydrogen (H):
(4 × 1.0 / 80.0) × 100 = (4 / 80) × 100 = 5.00% (Matches question data!) - Calculate percentage mass of Nitrogen (N):
(2 × 14.0 / 80.0) × 100 = (28 / 80) × 100 = 35.00% (Matches question data!) - Calculate percentage mass of Oxygen (O):
(3 × 16.0 / 80.0) × 100 = (48 / 80) × 100 = 60.00% (Matches question data!)
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.