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Haemoglobin has 0.33% iron by mass. If its molecular mass is about 67200 and the atomic mass of iron is 56, how many iron atoms are present in a single haemoglobin molecule?

1. 6
2. 1
3. 2
4. 4

Correct answer: 4

Solution

The percentage of iron by mass in haemoglobin indicates that for every 100 grams of haemoglobin, there are 0.33 grams of iron. Given the molecular mass of haemoglobin is approximately 67200 g/mol, the mass of iron in one mole of haemoglobin is calculated to be 0.33% of 67200, which equals about 221 grams. Dividing this by the atomic mass of iron (56 g/mol) shows that there are approximately 4 iron atoms in each haemoglobin molecule.

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