Suppose that you have 115 mL of a buffer that is 0.460 M in both benzoic acid ( C 6 H 5 COOH ) and its conjugate base ( C 6 H 5 COO − ) . Calculate the maximum volume of 0.390 M HCl that can be added to the buffer before its buffering capacity is lost.

The maximum volume of HCl that can be added before its buffering capacity is lost is 111.3 mL

Explanation:

We are given:

Concentration of buffer = 0.460 M

To calculate the pH of acidic buffer, we use the equation given by Henderson Hasselbalch:

       .....(1)

We are given:

= negative logarithm of acid dissociation constant of benzoic acid = 4.2

pH = ?

Putting values in equation 1, we get:

When the pH of the buffer changes by 1 unit, the buffering capacity is said to be lost.

pH change for loosing buffer capacity = [4.2 - 1] = 3.2

Calculating the ratio of conjugate base and its acid by using equation 1:

= 4.2

pH = 3.2

Putting values in equation 1, we get:

       ......(2)

For benzoic acid and its conjugate base:

Molarity of benzoic acid and its conjugate base = 0.460 M

Volume of solution = 115 mL

Putting values in equation 2, we get:

The chemical reaction for aniline and HCl follows the equation:

Let the moles of acid added to carry out the change is 'x' moles

Calculating the volume of acid added by using equation 2, we get:

Moles of acid added = 0.0434 moles

Molarity of solution = 0.390 M

Putting values in equation 2, we get:

Hence, the maximum volume of HCl that can be added before its buffering capacity is lost is 111.3 mL

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