Haemoglobin contains 0.334% of iron by weight. The molecular weight of haemoglobin is approximately 67200. The number of iron atoms (Atomic weight of Fe is 56) present in one molecule of haemoglobin is

0.33% of Iron by weight means 100 gms of heamoglobin has 0.33g of iron

100 g of heamoglobin contains iron = 0.33 g

67200 g of heamoglobin contains iron 
 

$\frac{0.33 \times 67200}{100}=221.76 \mathrm{gms} \mathrm{of} \mathrm{Fe}$

Atomic Wt. of Fe = 56

Number of Iron atoms = $\frac{\mathrm{Weight} \mathrm{of} \mathrm{iron} \mathrm{in} \mathrm{haemoglobin}}{\mathrm{Atomic} \mathrm{Wt}.}$

Number of Fe- atoms $=\frac{221.76}{56} =3.96 \simeq 4$