Question

The pH of ${{D}_{2}}O$ and ${{H}_{2}}O$ at 298K is:
(A) 7, 7
(B) 7.35, 7
(C) 7.0, 6.85
(D) 6.85, 7.35

Answer 1

Hint: To answer this question, concentrate on the bonding between oxygen and the deuterium or hydrogen. Stronger the covalent bonding, lower will be the dissociation rate and thus lower will be the corresponding cation and thus will be a weaker acid.

Complete step by step answer:
Before answering this question, let us understand the meaning of pH first.
pH is the measure of acidity of the solution. It is equal to the negative log of the molar concentration of hydronium ions released in the solution. We can write is as-
$pH=-\log ({{H}^{+}})$
Now, here we have water i.e. ${{H}_{2}}O$ and heavy water i.e. ${{D}_{2}}O$. pH is a temperature dependent property i.e. it varies with temperature. Here, the temperature given to us is 298K i.e. 25 degree Celsius that is the standard temperature. So, considering this we will explain the acidity of the two.
In pure water, the concentration of hydroxyl ions is equal to the concentration of hydronium ions which gives that the pH of pure water is 7. Now let’s discuss heavy water.
The O-D covalent bond is slightly stronger than the O-H polar bond and thus heavy water is less dissociated than water and the true concentration of ${{D}^{+}}$ ions is less than that of ${{H}^{+}}$ at a given temperature. Thus, water is slightly stronger acid than heavy water. So, the pH of water will be lower than heavy water.
Therefore, the correct answer is option (B) 7.35, 7.

Note: If a solution has pH 1-6 it is referred to as an acidic solution. If the pH is 7 the solution is neutral and if the pH is higher than7, then the solution is alkaline in nature. It is important to remember that if the number of hydroxyl ions is equal to the number of hydronium ions in a solution, then the solution is neutral and thus pH is 7. Similarly, an acidic solution has excess hydronium ions and a basic solution has excess hydroxyl ions.