Question

The haemoglobin from red corpuscles of most mammals contains approximately 0.33% of iron by weight. The molecular weight of haemoglobin is 67200. The number of iron atoms in each molecule of haemoglobin is:
A. 1
B. 2
C. 3
D. 4

Answer 1

Hint: The question deals with the concentration of iron atoms in terms of its mass in haemoglobin. We have to find the mole of iron atoms in one mole of haemoglobin, and then the number of iron atoms in each molecule of haemoglobin will be obtained.

Complete step by step answer:
Let us first write the meaning of the term 0.33% of iron by weight; it means that ‘0.33 grams of iron is present in 100 grams of haemoglobin’. Find the mass of iron present in 1 gram of haemoglobin. It will be obtained when it will be divided by 100 as$\dfrac{0.33}{100}$. Now convert the grams term in moles to reach the answer. Mole is defined as the ratio of given weight to molecular weight of that compound. Mathematically it is represented as
$\text{moles = }\dfrac{\text{given mass}}{\text{molar mass}}$. Molar mass of haemoglobin is 67200 grams which is given. If we multiply 1 gram of haemoglobin with 67200; then 67200 will be equal to 1 mole of haemoglobin as$\left( \dfrac{0.33}{100}\times 67200 \right)$. So, 1 mole of haemoglobin has 221.8 grams of iron. The atomic weight of iron is 56 grams. So, the moles of iron $\dfrac{221.8}{56}$ equals to 3.96 which is approx. 4. So, one mole of haemoglobin has 4 moles of iron. Similarly, one molecule of haemoglobin has 4 molecules of iron present in it, which matches option ‘d’.

Note: The ratio of moles of any two compounds will be the same as the ratio of the number of molecules present in those compounds. This is because moles are converted to molecules by multiplying moles with Avogadro's number. So,$\dfrac{\text{mole}{{\text{s}}_{1}}}{\text{mole}{{\text{s}}_{2}}}=\dfrac{\text{moles}\times {{\text{N}}_{\text{A}}}}{\text{moles}\times {{\text{N}}_{\text{A}}}}=\dfrac{\text{molecule}{{\text{s}}_{1}}}{\text{molecule}{{\text{s}}_{2}}}$.