Suppose that you have 185 ml of a buffer that is 0.260 M in both benzoic acid (C6H5COOH) and its conjugate base (C6H5COO-). Calculate the maximum volume of 0.170M HCl that can be added to the...

Suppose that you have 185 ml of a buffer that is 0.260 M in both benzoic acid (C6H5COOH) and its conjugate base (C6H5COO-). Calculate the maximum volume of 0.170M HCl that can be added to the buffer before its buffering capacity is lost.

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llltkl | College Teacher | (Level 3) Valedictorian

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The buffer capacity refers to the maximum amount of either strong acid or strong base that can be added to a buffer solution before a significant change in the pH will occur.  In a buffer consisting of a weak acid like benzoic acid and its salt with a strong base (i.e. its conjugate base), small amounts of acid that is added is consumed by the conjugate base present in the buffer. The resulting weak acid then dissociates to a small extent and hence a very small change in pH is observed. This is the mechanism of buffering.

When all the conjugate acid is exhausted in this manner, but addition of strong acid continues, a steep fall in pH is observed. The buffer is said to lose its buffering capacity at that point. The maximum amount of strong acid that can be added before losing its buffer capacity is equal to the total amount of conjugate base present in the buffer.  Similarly, the maximum amount of base that can be added is equal to the amount of weak acid present in the buffer.

`C_6H_5CO_2H(aq.) stackrel rarr larr C_6H_5CO_2^(-)(aq.) + H_3O^(+)(aq.)`

Amount of conjugate base (`C_6H_5CO_2^(-)` ) present in 185 ml buffer solution:

 = 185*0.26 mmol

 =0.0481 mol

The maximum amount of acid that can be added will be equal to the amount of conjugate base, 0.0481 mol.

Molarity of HCl solution that is being added: 0.170 M  

The maximum volume of 0.170M HCl that can be added:

=0.0481/0.170 lit.

=0.2829 lit.

=282.9 ml

Sources:

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