Question

You would like to make 100.00 mL of a 4.76 pH acetic acid/acetate buffer, in which the acid concentration is 0.070M. Write the reaction of the buffer with HCl solution. How many mL of 0.100 M HCl can react with the buffer before it is exhausted? And Write the reaction of the buffer with NaOH solution. How many mL of 0.100 M NaOH can react with the buffer before it is exhausted?

Answer 1

Reaction of buffer with HCl solution,

The pH of a buffer solution is governed by the following equilibrium

...........CH3COOH <==========> CH3COO^-   + H⁺

The buffer solution has a large excess of CH3COO^-  ions produced by complete ionization of sodium acetate.

.........................CH3COONa <=========> CH3COO^-   + Na⁺

Addition of HCl , Upon addition of HCl, the increase of H⁺ is counteracted by association with excess of acetate ions to form unionized CH3COOH . Thus the addition of H⁺ ions are neutralized and the pH of a buffer solution remains virtually unchanged . However, owing to increased concetration of acetic acid the equilibrium slightly to the right to increase H⁺ ions. This explains the marginal increase in pH of the buffer solution on addition of HCl.

Addition of NaOH , When NaOH is addedd to the buffer solution , the additional OH^- combine with H⁺ ion of the buffer to form water molecules. As a result the equilibrium shifts to the right to produce more and more H⁺ ions till practically all the OH^- are neutralized and the original buffer pH is restored. However a new equilibrium is set up in which the [CH3COOH] is lower than it was in the original buffer. Consequently [ H⁺ ] is also slightly less , and the pH is maginally higher than the buffer pH values.

Buffer capacity Even though a buffer solution is capable to resist a change in pH by addition of acids or bases, but there is a limit beyond which it fails to maintain its own pH (as briefed out above) and its pH changes sharply.It is numerically a value that quantifies the amount of H⁺ or OH^- that a solution is capable of neutralizing before the weak acid or conjugate base is used up.

Mathematically , buffer capacity is the number of moles of acid / base added to 1.00 Litre buffer solution for causing a unit change in pH.

With this view the volume of 0.1 M HCl required to exhaust the buffer capacity, call for following stepwiswise calculations-

Step 1 - The addition of HCl should change the pH of the solution this from 4.76 to 3.76 when its buffer would get exhausted.

Calculate moles of acid / L to cause this change = ( 0.070 x 3.76 ) / 4.76

.............................................................................= 0.0553 moles / L

Step 2 - Calculate the volume of 0.1 M HCl in ml. required to get this concentration in 100 ml of solution, using relation M1V1 = M2 V2

................................................= ( 0.0553 x 100 ) / 0.1

............................................... = 55.29 ml

Thus 55.3 ml of 0.10 M HCl is required to be added before buffer gets exhausted.

Similarly , calculate the volume of 0.10 M NaOH to exhaust the buffer of 100 ml. solution, when its pH changes from 4.76 to 5.76 as = 84.70 ml.