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Percentage by mass of an element = total relative atomic mass of that element in the formula ÷ Mr of the compound × 100. Free notes and 11 questions answered.
Percentage by Mass in Compounds is part of Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations in GCSE AQA Higher Chemistry.
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Percentage by mass of an element = total relative atomic mass of that element in the formula ÷ Mr of the compound × 100. Free notes and 11 questions answered.
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Use this lesson to stay inside Chemical measurements, conservation of mass and the quantitative interpretation of chemical equations while connecting the detail back to the wider Topic 3: Quantitative chemistry topic.
Read the core explanations from Percentage by Mass in Compounds before testing yourself from memory.
You buy a bag of fertiliser labelled "20% nitrogen". How do you know if you're getting what you paid for? The answer lies in percentage by mass calculations.
Percentage by mass tells us exactly how much of each element is present in any compound. Whether you have 1 gram or 1 tonne of calcium carbonate, the percentage of calcium stays the same. This makes it perfect for quality control, medicine dosages, and industrial processes.
Think of it like a pizza. If you want to know what percentage of the pizza is cheese, you'd weigh the cheese and divide by the total pizza weight. Percentage by mass works exactly the same way with atoms in compounds.
The beauty of this calculation is that it uses relative masses from the periodic table, not actual masses. This means your answer works for any amount of the compound - whether you have a few molecules or several kilograms.
Every percentage by mass calculation follows the same three-step process. Let's master this method using calcium carbonate (CaCO₃) as our example.
Worked example: Finding percentage by mass of calcium in CaCO₃
Calculate the percentage by mass of calcium in calcium carbonate, CaCO₃. [3 marks]
Answer: 40%
Notice how we got exactly 40% - this is because the relative formula mass of CaCO3 is 100, which makes the arithmetic especially straightforward in this example. In exam questions, the numbers often work out cleanly like this.
Students lose marks on percentage by mass questions for predictable reasons. Here are the traps examiners set and how to avoid them.
Trap 1: Forgetting to multiply by the number of atoms. In H₂SO₄, there are 2 hydrogen atoms and 4 oxygen atoms. Many students use H = 1 and O = 16 instead of H = 2 and O = 64.
Trap 2: Using the wrong mass in the numerator. If asked for the percentage of oxygen in CaCO₃, use 48 (the mass of all oxygen atoms), not 16 (the mass of one oxygen atom).
Worked example: Percentage by mass with multiple atoms
Calculate the percentage by mass of oxygen in sulfuric acid, H₂SO₄. [3 marks]
Answer: 65.3%
Percentage by mass questions are worth 3-4 marks and follow a predictable mark scheme. Here's how to guarantee full marks every time.
The mark scheme typically awards: [1 mark] for correct relative formula mass, [1 mark] for correct substitution into the formula, [1 mark] for the correct final answer with percentage sign.
Master this method and you'll never lose marks on percentage by mass questions. The key is systematic working - follow the same steps every time, show all your working, and double-check your arithmetic.
Exam precision note: write the full method in order: find the relative mass of the chosen element, find the relative formula mass of the whole compound, then divide and multiply by 100. If the answer is neat, still show the method because the marks are for the process as well as the final percentage.
Percentage by mass depends on relative formula mass, so you are always comparing part of the compound with the whole compound. This is why compounds with the same element can give different percentages: the other atoms in the formula change the total Mr and therefore change the fraction represented by that element.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Percentage by mass of an element = total relative atomic mass of that element in the formula ÷ Mr of the compound × 100. For calcium in CaCO₃, Mr = 40 + 12 + (3 × 16) = 100, so % Ca = 40 ÷ 100 × 100 = 40%.
The relative formula mass of calcium carbonate, CaCO₃, is 100. Using Ca = 40, C = 12 and O = 16, the calculation is 40 + 12 + (3 × 16) = 40 + 12 + 48 = 100.
The relative formula mass of sulfuric acid, H₂SO₄, is 98. It has 2 hydrogen atoms, 1 sulfur atom and 4 oxygen atoms, so with H = 1, S = 32 and O = 16 the calculation is (2 × 1) + (1 × 32) + (4 × 16) = 2 + 32 + 64 = 98.
Calcium makes up 40% of calcium carbonate by mass. With Ca = 40, C = 12 and O = 16, the Mr of CaCO₃ is 40 + 12 + (3 × 16) = 100, so the percentage of calcium is 40 ÷ 100 × 100 = 40%.
Oxygen makes up 48% of calcium carbonate by mass. CaCO₃ has 3 oxygen atoms, so the mass of oxygen is 3 × 16 = 48, not 16, and dividing by the Mr of 100 and multiplying by 100 gives 48%.
Percentage by mass is the mass of a specific element in a compound divided by the total mass of the compound, multiplied by 100. It shows how much of each element is present in the compound.
The percentage by mass of oxygen in sulfuric acid, H₂SO₄, is 65.3%. Mr = (2 × 1) + 32 + (4 × 16) = 98, and the mass of the four oxygen atoms is 64, so % O = 64 ÷ 98 × 100 = 65.3%.
The most common trap in percentage by mass questions is forgetting to multiply by the number of atoms of the element. For oxygen in CaCO₃, the mass to use is 48 for all three oxygen atoms, not 16 for one oxygen atom.
Percentage by mass does not depend on sample size because it uses relative masses from the periodic table, not actual masses. Whether there is 1 gram or 1 tonne of calcium carbonate, the percentage of calcium stays the same.
Compounds with the same element give different percentages by mass because the other atoms in the formula change the total Mr. Percentage by mass always compares part of the compound with the whole compound, so a different total changes the fraction represented by that element.
Percentage by mass is used for quality control, medicine dosages and industrial processes. It can check whether a bag of fertiliser labelled '20% nitrogen' really has the nitrogen it says, and it can be used to evaluate the purity of compounds.