Question

# A buffer is a mixed solution of a weak acid or base, combined with its conjugate....

A buffer is a mixed solution of a weak acid or base, combined with its conjugate. Note that this can be understood essentially as a common-ion problem: The conjugate is a common ion added to an equilibrium system of a weak acid or base. The addition of the conjugate shifts the equilibrium of the system to relieve the stress of the added concentration of the common ion. In a solution consisting of a weak acid or base, the equilibrium shift also results in a pH shift of the system.

It is the presence of the common ion in the system that results in buffering behavior, because both added H+ or OH− ions can be neutralized.

Part D

What is the pH change of a 0.300 M solution of citric acid (pKa=4.77) if citrate is added to a concentration of 0.145 M with no change in volume?

Express the difference in pH numerically to two decimal places.

The initial pH of the citric acid solution can be calulated using the ICE table

C6H8O7 C6H87O7- + H+

initial 0.3 0 0

change -x +x +x

equilibrium 0.3-x x x

Ka = x2/0.3-x   pKa = -log(Ka); Ka = 10-pka = 10-4.77 = 1.69 x 10-5

1.69 x 10-5 = x2/0.3-x    (x in the denominatore will be small and can be ignored with respect to 0.3)

1.69 x 10-5 x 0.3 = x2

x2 = 5.09 x 10-6

x = 0.00226 M

This is the H+ ion concentration

pH = -log H+

pH = -log 0.00226

pH = 2.64 This is the pH of 0.3 M citric acid solution

pH of the buffer containing citric acid and citrate can be calculated using henderson hasselbalch equation

pH = pKa + log base/acid

pH = 4.77 + log 0.145/0.3

pH = 4.77 - 0.316

pH = 4.45

Change in pH is 4.45 - 2.64 = 1.81

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