Relative Formula Mass — GCSE Chemistry Revision
Revise Relative Formula Mass for GCSE Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP.
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Go to Conservation of MassWhat is Relative Formula Mass?
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance compared to the mass of a carbon-12 atom.
Board notes: Calculating relative formula mass is a fundamental skill required for many other calculations in chemistry, such as mole calculations. It is tested by all exam boards at all tiers.
Step-by-step explanationWorked example
To find the relative formula mass of magnesium carbonate (MgCO₃): Ar of Mg = 24, Ar of C = 12, Ar of O = 16. Mr(MgCO₃) = 24 + 12 + (3 x 16) = 24 + 12 + 48 = 84.
Mini lesson for Relative Formula Mass
1. Understand the core idea
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance compared to the mass of a carbon-12 atom.
Can you explain Relative Formula Mass without copying the notes?
2. Turn it into marks
To find the relative formula mass of magnesium carbonate (MgCO₃): Ar of Mg = 24, Ar of C = 12, Ar of O = 16. Mr(MgCO₃) = 24 + 12 + (3 x 16) = 24 + 12 + 48 = 84.
Underline the method, evidence, or command-word move that would earn credit in GCSE Quantitative Chemistry.
3. Fix the likely mark leak
Watch for this mistake: Confusing relative atomic mass (Ar) with relative formula mass (Mr). Ar is for a single atom, while Mr is for a whole compound.
Write one correction rule before doing another practice question.
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Relative Formula Mass practice questions
These are original StudyVector questions for revision practice. They are not official exam-board questions.
Question 1
In one GCSE sentence, explain what Relative Formula Mass is testing.
Answer: The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance compared to the mass of a carbon-12 atom.
Mark focus: Precise definition and topic focus.
Question 2
A Relative Formula Mass question uses an unfamiliar context. What should the answer do before adding detail?
Answer: It should name the process, variable, equation, particle model, or evidence being tested, then explain the result using precise scientific vocabulary.
Mark focus: Method selection and command-word control.
Question 3
A student makes this mistake: "Confusing relative atomic mass (Ar) with relative formula mass (Mr). Ar is for a single atom, while Mr is for a whole compound." What should their next repair task be?
Answer: Do one Relative Formula Mass question and review the mistake type.
Mark focus: Error correction and next-step practice.
Relative Formula Mass flashcards
Core idea
What is the main idea in Relative Formula Mass?
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance compared to the mass...
Common mistake
What mistake should you avoid in Relative Formula Mass?
Confusing relative atomic mass (Ar) with relative formula mass (Mr). Ar is for a single atom, while Mr is for a whole compound.
Practice
What is one useful practice task for Relative Formula Mass?
Answer one Relative Formula Mass question and review the mistake type.
Exam board
How should you use board notes for Relative Formula Mass?
Calculating relative formula mass is a fundamental skill required for many other calculations in chemistry, such as mole calculations. It is tested by all exam boards at all tiers.
Common mistakes
- 1Confusing relative atomic mass (Ar) with relative formula mass (Mr). Ar is for a single atom, while Mr is for a whole compound.
- 2Forgetting to multiply the Ar of an element by the number of atoms of that element in the formula (e.g., the '2' in H₂O).
- 3Making simple arithmetic errors when adding up the masses. Always double-check your calculations.
Relative Formula Mass exam questions
Exam-style questions for Relative Formula Mass with mark-scheme style solutions and timing practice. Aligned to AQA, Edexcel, OCR, WJEC, Eduqas, CCEA, Cambridge International (CIE), SQA, IB, AP specifications.
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Relative Formula Mass
Core concept
The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in its chemical formula. It is a measure of the mass of one formula unit of a substance comp…
Frequently asked questions
What are the units for relative formula mass?
Relative formula mass is a ratio, so it has no units. It is just a number.
Where do I find the relative atomic masses?
Relative atomic masses for all elements are given in the periodic table, usually as the larger of the two numbers shown for each element.