Question

Hemoglobin 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 Hemoglobin is:
(A) 5 atoms
(B) 4 atoms
(C) 2 atoms
(D) None of these

Answer 1

First of all, we will calculate the weight of Fe present in 1 molecule of haemoglobin using the weight percentage of Fe and weight of 1 haemoglobin molecule given in the question. We will then use a unitary method to calculate the total number of Fe atoms associated with the obtained weight.

Complete step by step solution:
-First of all, let us see what haemoglobin is.
Haemoglobin is an iron-containing protein present in the blood of many animals—in the red blood cells or erythrocytes of vertebrates. It transports oxygen to the tissues via blood vessels. Haemoglobin forms an unstable reversible bond with oxygen which is extremely essential. In the oxygenated state, it is called oxyhemoglobin and is bright red in colour; in the reduced state, it is purplish-blue.
-Given, Molecular weight of haemoglobin = 67200.
-Hemoglobin contains 0.334% iron by weight this means that 0.334% of 67200 is the weight of iron present in haemoglobin.
-We know that the mass percentage is calculated as the mass of a component divided by the total mass of the mixture, multiplied by 100.
Mathematically it can be written as:
$\% mass = \dfrac{{wt.component}}{{wt.mixture}} \times 100$______________(1)
We have the mass % of iron and we have the total weight of the haemoglobin, so we will use the above equation to calculate the amount of iron present in 1 molecule of haemoglobin:
$0.334 = \dfrac{W}{{67200}} \times 100$
$W$ = $0.334 \times \dfrac{{67200}}{{100}}$
$= 224.448 g$
-We know that the molar mass of 1 atom of iron = 56 g/mol
And the total weight of Fe in a molecule of haemoglobin is = 224.448 g
We will now use the unitary method to calculate the number of atoms of Fe associated with 224.448 g of Fe:
$56 g$ → $1$ atom of Fe
$1 g$ → $\dfrac{1}{{56}}$ atoms of Fe
$224.448 g$ → $\dfrac{1}{{56}} \times 224.448$
= 4.008 atoms of Fe
Hence we can say that 1 molecule of haemoglobin will contain 4 atoms of iron.

Hence, the correct answer is: (B) 4 atoms.

Additional information:
When we breathe in carbon monoxide, the CO also binds to haemoglobin and forms a stable compound carboxyhemoglobin. It binds so strongly that it keeps oxygen from binding to haemoglobin. Over time, if we breathe enough carbon monoxide, it binds to all of the haemoglobin in our body, and we no longer have enough oxygen in our blood. This may be fatal.

Note: Two-thirds of the body's iron is present in circulating red blood cells as haemoglobin. Each gram of haemoglobin contains 3.47 mg of iron. Males of average height have about 4 grams of iron in their body, females about 3.5 grams while children will usually have 3 grams or less.