Question

A titration involves adding a reactant of known quantity to a solution of an another reactant while monitoring the equilibrium concentrations. This allows one to determine the concentration of the second reactant. The equation for the reaction of a generic weak acid HA with a strong base is

HA(aq)+OH−(aq)→A−(aq)+H2O(l)

A certain weak acid, HA, with a Ka value of 5.61×10−6, is titrated with NaOH.

A: A solution is made by titrating 7.00 mmol (millimoles) of HA and 2.00 mmol of the strong base. What is the resulting pH?

B: More strong base is added until the equivalence point is reached. What is the pH of this solution at the equivalence point if the total volume is 46.0 mL ?

Answer 1

ANSWER:

K_a = 5.61 x 10^-6

(A) The reaction can be written as:

Number of milimoles of acid = 5.0 mmol

Number of milimoles of conjugate base = 2.0 mmol

According to Henderson-Hesselbaltch equation:

pH = -log(K_a) - log{[HA]/[A^-]}

pH = -log(5.61 x 10^-6) - log{[5.0 mmol]/[2.0 mmol]}

pH = 5.251 - 0.398 = 4.853

Hence, the resulting pH is 4.853.

(B) At equivalence point of titration, the number of moles of acid and base is equal. So, [HA]/[A^-] = 1 and hence, log{[HA]/[A^-]} = 0. That means:

pH = pK_a = -log(K_a)

pH = -log(5.61 x 10^-6)

pH = 5.251

Hence, the pH of this solution at the equivalence point is 5.251.