In water, the Fe^2+ ion bonds with water to form Fe(H2O)6^2+. In haemoglobin in the blood, five of the water molecules have been displaced. In the presence of air, the last water molecule is displaced by oxygen to give oxyhaemoglobin. In the presence of carbon monoxide, the oxygen is displaced to give carboxyhaemoglobin.
Which of the following shows the correct order of the strength of the bonding to the Fe^2+ ion (weakest first)?

Answer:

23) E

The order is clearly given by the paragraph.

The third sentence says that in the presence of air, the last water molecule is displaced by oxygen.

So water is weaker than oxygen.

In the last sentence, it says that in the presence of carbon monoxide, oxygen is displaced. So oxygen is weaker than carbon monoxide. Therefore, water must also be weaker than carbon monoxide.

We get the order:

water, oxygen, carbon monoxide.