If I have 10 g of two different substances, does the one with bigger Mr have more moles?

No, the opposite. Since n = m / Mr, for the same mass a substance with a larger Mr will have fewer moles. 10 g of H2 (Mr = 2) is 5.0 mol, while 10 g of CaCO3 (Mr = 100) is only 0.10 mol.

How should I think about the relationship between Mr and moles?

Think of Mr as the mass of one mole. A substance with a large Mr needs more grams to make one mole. So a fixed mass of grams gives you fewer moles of a high-Mr substance than a low-Mr substance.

A Bigger Mr Does Not Mean More Moles

Why IGCSE Chemistry students wrongly think that a higher relative formula mass means more moles of substance, how mass and moles relate inversely for a fixed mass, and how to apply n = m / Mr correctly on 0620.

Published by IGCSEChemistry.com.my

Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).

Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.

What students typically write

“CaCO3 has a bigger Mr than NaOH, so 10 g of CaCO3 has more moles.” Or: “Bigger molecules have more moles.”

Students see a larger number (the Mr) and associate it with “more”. They forget that n = m / Mr is a division: a bigger denominator gives a smaller result.

Why it is wrong

The formula n = m / Mr tells you that for a fixed mass, the number of moles is inversely proportional to the relative formula mass. A substance with a large Mr has heavy particles, so fewer of them fit into the same mass.

Example: 10 g of hydrogen gas (Mr = 2) contains 10 / 2 = 5.0 mol. But 10 g of calcium carbonate (Mr = 100) contains 10 / 100 = 0.10 mol. The hydrogen has 50 times more moles despite having the same mass, precisely because each hydrogen molecule is 50 times lighter than each formula unit of calcium carbonate.

Think of it like carrying fruit: 10 kg of grapes gives you far more individual pieces than 10 kg of watermelons, because each grape is much lighter than each watermelon.

What the mark scheme actually wants

Calculation questions test whether you can apply n = m / Mr correctly. The mark scheme does not reward intuition about “bigger Mr means more” because that intuition is wrong.

A typical question: “Which contains more moles: 6.0 g of carbon (Ar = 12) or 6.0 g of magnesium (Ar = 24)?” [2]

Marking points:

Carbon: n = 6.0 / 12 = 0.50 mol (1 mark)
Magnesium: n = 6.0 / 24 = 0.25 mol, so carbon has more moles (1 mark)

Worked example: wrong vs right

Question: A student has 20 g of sodium hydroxide (Mr = 40) and 20 g of sulfuric acid (Mr = 98). Which contains more moles? [2]

Wrong answer: “Sulfuric acid has a bigger Mr (98 > 40), so it has more moles.” This scores 0/2. The logic is backwards.

Right answer: “NaOH: n = 20 / 40 = 0.50 mol. H2SO4: n = 20 / 98 = 0.204 mol. The sodium hydroxide contains more moles (1)(1).” This scores 2/2. The calculation shows the correct inverse relationship.

How to avoid this mistake

Always calculate. Never guess based on the size of Mr. The formula n = m / Mr is fast and leaves no room for this error.

If you want a quick intuition check, remember: Mr tells you “how many grams per mole”. A bigger Mr means heavier particles, so fewer moles per gram. Fewer, not more.

For full practice with mole calculations, see the mole and Avogadro constant and reacting mass calculations.

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