Question

What volume of $0.2$ M sodium formate is to be added to $100$ ml of $0.1$ M formic acid to form a buffer with $pH=4$ . [for formic acid is $3.80$ ].

Answer 1

Hint : We know that the buffer solution is basically an aqueous solution which resists the change in pH when a small amount of strong acid or base is added and formed by the mixture of weak acid and its conjugate base or its salt and vice-versa. The above question can be solved by the Henderson-Hasselbach equation.

Complete Step By Step Answer:
As we know, the buffer is an aqueous solution that consists of a mixture of a weak acid and its salt (acid buffer) or a weak base with its salt (basic buffer). Its pH changes very little when a small amount of strong acid or base is added to it and is thus used to prevent a solution’s pH change. Buffer solutions are used for a wide range of chemical applications. Blood is one example of a buffer solution found in nature. Solutions with the stable concentration of hydrogen ions and thus typically with no change in pH which is almost independent of dilution and which change very little with small additions of a strong acid or alkali are called buffers. It can also be described in simple terms as a solution that prevents any pH change when a small amount of a strong acid or a strong base is applied to it, is called a buffer solution or simply as a buffer. Both buffers have acidity and alkalinity balance.
First, let us consider the volume of $0.20\text{ }M$ sodium formate added to be x. Now, with the help of mole concept,
Now, from the Henderson’s equation we have;
$pH=pKa+log\left[ \dfrac{HCOONa}{HCOOH} \right]$ here $pH$ is $4$ and $pKa$ is $3.80;$
Put these values in the Henderson’s equation
$4.0=3.80+log\left[ \dfrac{x\times 0.20}{100\times 0.1} \right]$
On further solving we get;
$0.2=log0.02x$
$\Rightarrow x=79.24\text{ }mL$ .

Note :
Remember that the acidity of the solution in which they occur affects a lot of chemical reactions. The pH of the reaction medium must be controlled for a given reaction to occur or to occur at a suitable rate. This control is provided by buffer solutions, which are solutions that preserve a certain pH.