Question

A solution is prepared by dissolving 0.23 mol of hypochlorous acid (HClO) and 0.27 mol of sodium hypochlorite (NaHClO) in enough water to make 1.00 L of solution. The addition of 0.05 mol of HCl to this buffer solution causes the pH to drop slightly. The pH does not decrease drastically because the HCl reacts with the ________ present in the buffer solution. The K_a of hypochlorous acid is 1.36 × 10-3.

	a.	H3O+
	b.	H2O
	c.	hypochlorous acid
	d.	hypochlorite ion
	e.	This is a buffer solution: the pH does not change at all upon addition of acid or base.

Answer 1

The property of a buffer solution is to resist the change in pH on addition of acid or base.

If acid is added, it reacts with the base to give the weak acid already present in the solution and neutral salt.

For example if we add HCl in the solution, it will react with hypochlorite ion to give hypochlorous acid and sodium chloride (neutral salt) . And we know that the dissociation constant of weak acid is very less so the concentration of H+ ion will not increase much hence there will be a slight change in pH.

Let's Lets us see by solving what is the change in pH.

So we saw that before addition of hydrochloric acid pH of the solution was 2.80 and after the addition of hydrochloric acid pH of the buffer solution is 2.97. Hence pH has changed by 0.17 only.

So, pH does not decrease drastically because the HCl reacts with the HYPOCHLORITE ION present in the buffer solution.  

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