Question

Assertion: Air dissolved in water contains a lower percentage of oxygen than contained in air.
Reason: The solubility of oxygen in water is 33%.
[A] Both assertion and reason are correct and the reason is the correct explanation for assertion.
[B] Both assertion and reason are correct but the reason is not the correct explanation for the assertion.
[C] Assertion is incorrect but reason is correct.
[D] Both assertion and reason are incorrect.

Answer 1

Hint: We know that air is a mixture of certain gases and it contains oxygen about 21%. When we dissolve air in water, a strong inter-nuclear force of attraction comes into action. It affects the solubility of oxygen in water. Use this to answer the question.

Complete step by step solution:
We know that air is a mixture of several gases like nitrogen, oxygen, carbon dioxide and other noble gases. We also know that nitrogen is the most abundant gas in air with a percentage of 78% and oxygen consists of 21% of air.
Now let us discuss the factors affecting dissolution of air in water.
We know that the amount of any gas dissolving in any solvent is known as its solubility. The solubility of air dissolving in water depends upon the temperature and the pressure.
It is directly proportional to pressure and inversely proportional to temperature. This means that with increase in pressure, solubility increases and with increases in temperature, solubility decreases.
However when dissolved in water, the oxygen content increases to 33-36%.
In water, we see hydrogen bonding between oxygen and water and thus it results in increased solubility.
Therefore, the solubility of oxygen is more in water than in air.
Now, the assertion given to us says air dissolved in water contains a lower percentage of air, but we have seen in the above discussion that ordinary air contains 21% of oxygen and air dissolved in water contains 33-36% oxygen. Therefore, the assertion is incorrect.
And the reason given to us says that the solubility of oxygen in water is 33%, which we can understand from the above discussion that it is correct.

Therefore, the correct answer is option [C] Assertion is incorrect but reason is correct.

Note: Here we have mentioned hydrogen bonding as the reason for increased solubility. Hydrogen bonding is the strongest intermolecular force. It is a special case of a dipole-dipole interaction which is the result of attractive force between a hydrogen atom of a covalent compound containing a highly electronegative group and an electronegative atom like oxygen, nitrogen, fluorine etc. For example hydrogen bonding between water molecules and oxygen as we have mentioned in the above discussion.