Past Paper Practice: Isotope Calculations
/40 marks
Topic 2.3.2: Calculating Relative Atomic Mass from Isotopic Abundance
Exam Focus: Papers 2 & 4 | Mathematical Precision Required
Examiner's Mathematical Protocol
- • Show ALL working - Method marks are awarded even if final answer is wrong
- • Units: Aᵣ has no units (it's a ratio)
- • Significant Figures: Match the data given (usually 2-3 s.f.)
- • Common Error: Forgetting to divide by 100 at the end
- • Reverse Logic: If Aᵣ is closer to one isotope, that isotope is MORE abundant
Section A: Theory & Formula Recall
Questions 1-5 | 7 marks
State the formula for calculating the relative atomic mass (Aᵣ) of an element from the isotopic abundances of its isotopes.
Answer space - Complete your working here
Define the term "isotope" and explain why isotopes of the same element have different masses.
Answer space - Complete your working here
The mass spectrum of an element shows peaks at mass numbers 24, 25, and 26. What information do these peaks provide?
Answer space - Complete your working here
Explain why the relative atomic mass of chlorine (35.5) is not a whole number, even though all isotopes have whole number mass numbers.
Answer space - Complete your working here
In a calculation of relative atomic mass, why must the percentages of all isotopes add up to 100%?
Answer space - Complete your working here
Section B: Standard Calculations
Questions 6-10 | 17 marks
A sample of chlorine contains 75% ³⁵Cl and 25% ³⁷Cl. Calculate the relative atomic mass of chlorine. Show your working.
Answer space - Complete your working here
Bromine has two isotopes: ⁷⁹Br (50.5%) and ⁸¹Br (49.5%). Calculate the relative atomic mass of bromine.
Answer space - Complete your working here
Magnesium has three isotopes: ²⁴Mg (79%), ²⁵Mg (10%), and ²⁶Mg (11%). Calculate the relative atomic mass of magnesium.
Answer space - Complete your working here
A sample of neon contains 90% ²⁰Ne and 10% ²²Ne. Calculate the relative atomic mass.
Answer space - Complete your working here
Silicon has three isotopes with the following data: ²⁸Si: 92.2% ²⁹Si: 4.7% ³⁰Si: 3.1% Calculate the relative atomic mass of silicon.
Answer space - Complete your working here
Section C: Reverse Calculations (Higher-Order)
Questions 11-15 | 16 marks
Element X has two isotopes with mass numbers 10 and 12. The relative atomic mass of X is 10.4. Which isotope is more abundant? Explain your reasoning.
Answer space - Complete your working here
Copper has two isotopes: ⁶³Cu and ⁶⁵Cu. The relative atomic mass of copper is 63.5. Calculate the percentage abundance of ⁶³Cu.
Answer space - Complete your working here
Boron has two isotopes: ¹⁰B and ¹¹B. The relative atomic mass is 10.8. Without calculation, state which isotope is more abundant.
Answer space - Complete your working here
Gallium has two isotopes: ⁶⁹Ga and ⁷¹Ga. The relative atomic mass is 69.8. Calculate the percentage abundance of each isotope.
Answer space - Complete your working here
Rubidium has two isotopes: ⁸⁵Rb and ⁸⁷Rb. In a sample, 72% of the atoms are ⁸⁵Rb. (a) Calculate the percentage of ⁸⁷Rb. (b) Calculate the relative atomic mass of rubidium.
Answer space - Complete your working here
📐 Essential Formula Reference
Aᵣ = Σ(isotope mass × % abundance) / 100
Weighted average of all isotopic masses
Example: Chlorine (75% ³⁵Cl, 25% ³⁷Cl)
Aᵣ = (35 × 75 + 37 × 25) / 100 = 3550 / 100 = 35.5
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