Question

When 0.5 mole of NaOH were added to one litre of a buffer solution, its pH changes from 5.70 to 5.85. Find the buffer capacity (nearest integer).

Answer 1

Here, the moles of base in one litre buffer solution and change of pH is provided. So, we have to use the formula of buffer capacity. The formula is,
${\rm{Buffer}}\,{\rm{capcity}} = \dfrac{{{\rm{Moles}}\,{\rm{of}}\,{\rm{acid}}\,{\rm{or}}\,{\rm{base}}\,{\rm{added}}\,{\rm{to}}\,\,{\rm{the}}\,{\rm{solution}}}}{{{\rm{pH}}\,{\rm{change}}}}$

Complete step by step answer:
Let’s first understand the term buffer capacity. It is defined as the moles of base or acid needed to change the pH of a solution by one. Chemically buffer capacity is represented by $\beta$.

Now, come to the question. The Moles of NaOH is given as 0.5 and the change of pH takes place from 5.70 to 5.85.

Now, we have to put the values in the formula of buffer capacity.

${\rm{Buffer}}\,{\rm{capcity}} = \dfrac{{{\rm{Moles}}\,{\rm{of}}\,{\rm{acid}}\,{\rm{or}}\,{\rm{base}}\,{\rm{added}}\,{\rm{to}}\,\,{\rm{the}}\,{\rm{solution}}}}{{{\rm{pH}}\,{\rm{change}}}}$

$\Rightarrow {\rm{Buffer}}\,{\rm{capacity}} = \dfrac{{0.5}}{{5.85 - 5.70}}$

$\Rightarrow {\rm{Buffer}}\,{\rm{capacity}} = 3.33 \approx 3$

Hence, buffer capacity is 3.

Additional Information:

Buffer solution is a solution that resists the change in value of pH by adding a small amount of base and acid to it. There are various types of buffers, such as, simple buffers, mixed buffers, acidic buffers and basic buffers.

Simple buffers are the salts of weak acids and weak bases. Some examples are, ammonium nitrate and ammonium acetate.

Mixed buffers are the mixture of two components. These are either acidic or basic

Acidic buffers consist of equimolar quantities of weak acid and its salt with a strong base.

Basic buffers consist of equimolar quantities of weak base and its salt with a strong acid.

Note: The capacity of a buffer solution to resist change of pH is termed as buffer action. A buffer has reverse acidity which it uses against a base and a reverse basicity which it employs against an acid. Thus, there is no change of pH.