Strong base is dissolved in 625 mL of 0400 M weak acid Ka 3

Strong base is dissolved in 625 mL of 0.400 M weak acid (Ka = 3.15 × 10-5) to make a buffer with a pH of 3.93. Assume that the volume remains constant when the base is added.

Calculate the pKa value of the acid and determine the number of moles of acid initially present.

When the reaction is complete, what is the concentration ratio of conjugate base to acid?

How many moles of strong base were initially added?

Solution

moles of weak acid = 625 x 0.400 / 1000 = 0.25

Ka = 3.15 x 10^-5

pKa = -log Ka = -log (3.15 x 10^-5 )

pKa = 4.50

number of moles of acid initially = 0.250 mol

pH = pKa + log [conjugate base / acid]

3.93 = 4.50 + log [conjugate base / acid]

[conjugate base / acid] = 0.269

conjugate base = 0.250 x 0.269 = 0.0673

moles of strong base added = 0.0673 mol