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AQA GCSE · Question 08.3 · Quantitative chemistry
Calculate the relative formula mass (Mᵣ) of Fe₂O₃.\nRelative atomic masses (Aᵣ): O = 16, Fe = 56
Calculate the relative formula mass (Mᵣ) of Fe₂O₃.\nRelative atomic masses (Aᵣ): O = 16, Fe = 56
How to approach this question
1. Identify the number of atoms of each element in the formula Fe₂O₃. There are 2 iron (Fe) atoms and 3 oxygen (O) atoms.\n2. Find the relative atomic mass (Aᵣ) for each element from the information given (Fe = 56, O = 16).\n3. Multiply the number of atoms of each element by its Aᵣ.\n4. Add the results together to get the total relative formula mass (Mᵣ).
Full Answer
Mᵣ(Fe₂O₃) = (2 × Aᵣ(Fe)) + (3 × Aᵣ(O))\nMᵣ(Fe₂O₃) = (2 × 56) + (3 × 16)\nMᵣ(Fe₂O₃) = 112 + 48\nMᵣ(Fe₂O₃) = 160
The relative formula mass (Mᵣ) is the sum of the relative atomic masses (Aᵣ) of all the atoms in the formula.\nThe formula is Fe₂O₃.\nThis contains:\n- 2 atoms of Iron (Fe)\n- 3 atoms of Oxygen (O)\n\nUsing the given Aᵣ values:\nAᵣ of Fe = 56\nAᵣ of O = 16\n\nMᵣ of Fe₂O₃ = (2 × Aᵣ of Fe) + (3 × Aᵣ of O)\nMᵣ = (2 × 56) + (3 × 16)\nMᵣ = 112 + 48\nMᵣ = 160
Common mistakes
✗ Multiplying the number of atoms incorrectly (e.g., 2 × 16 and 3 × 56).\n✗ Adding the Aᵣ values without multiplying by the number of atoms (56 + 16).
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