Haemoglobin contains $$0.33\% $$  of iron by weight. The molecular weight of haemoglobin is approximately $$67200 g.$$  The number of iron atoms $$\left( {{\text{at}}{\text{.}}\,{\text{weight of}}\,Fe\,{\text{is}}\,\,56} \right)$$     present in one molecule of haemoglobin are

Solution :
$$\because \,0.33\% \,$$  of iron by weight means $$100g$$  of haemoglobin has $$0.33 g$$  of iron
$$100 g$$  of haemoglobin contains iron $$= 0.33 g$$
∴ $$67200 g$$   of haemoglobin contains iron
$$\eqalign{ & = \frac{{0.33 \times 67200}}{{100}}g \cr & = 221.76\,g\,{\text{of}}\,Fe \cr & {\text{Number of }}Fe{\text{ - }}atoms \cr & = \frac{{221.76}}{{56}} \cr & = 3.96 \approx 4 \cr} $$