Question

Haemoglobin contains 0.25% iron by mass. The molecular mass of haemoglobin is 89600. Calculate the number of iron atoms per molecule of haemoglobin.

Answer 1

Hint: The atomic mass of Iron (Fe) is 56u and the mass percentage of substance is defined as the number of grams of the given element in 100 grams of the given compound.

Step-by-Step Solution:
Before we move on to the solution to this particular question, let us first talk about what haemoglobin really is and its biological significance.
Haemoglobin (Hb) is a protein found in the red blood cells that carries oxygen in your body and gives blood its red colour. Haemoglobin levels vary from person to person. Haemoglobin in blood carries oxygen from the lungs or gills to the rest of the body (i.e. the tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 20 grams of haemoglobin in every 100 ml of blood.
Each haemoglobin molecule is made up of four heme groups surrounding a globin group, forming a tetrahedral structure. Heme, which accounts for only 4 percent of the weight of the molecule, is composed of a ring-like organic compound known as a porphyrin to which an iron atom is attached. It is the iron atom that binds oxygen as the blood travels between the lungs and the tissues.
Let us now come to the mathematical solution of this question.
∵100g haemoglobin has iron = 0.25g (by definition of mass percentage) ∴89600u haemoglobin has iron =0.25×89600100 = 224u Fe 1 molecule of haemoglobin has 224u Fe = (224/56) u atom of Fe = 4 atoms of Fe ∴1 molecule of haemoglobin = 4 atoms of Fe

Note: It is the iron atom that binds oxygen as the blood travels between the lungs and the tissues. There are four iron atoms in each molecule of haemoglobin, which accordingly can bind four atoms of oxygen.