Question

Calculate pH for a strong acid/strong base titration.

Determine the pH during the titration of 39.8 mL of 0.334 M HI by 0.334 M KOH at the following points:

(a) Before the addition of any KOH-?

(b) After the addition of 19.9 mL of KOH-?

(c) At the equivalence point-?

(d) After adding 47.8 mL of KOH-?

Answer 1

a)when 0.0 mL of KOH is added

Given:

M(HI) = 0.334 M

V(HI) = 39.8 mL

M(KOH) = 0.334 M

V(KOH) = 0 mL

mol(HI) = M(HI) * V(HI)

mol(HI) = 0.334 M * 39.8 mL = 13.2932 mmol

mol(KOH) = M(KOH) * V(KOH)

mol(KOH) = 0.334 M * 0 mL = 0 mmol

We have:

mol(HI) = 13.29 mmol

mol(KOH) = 0 mmol

0 mmol of both will react

remaining mol of HI = 13.29 mmol

Total volume = 39.8 mL

[H+]= mol of acid remaining / volume

[H+] = 13.29 mmol/39.8 mL

= 0.334 M

use:

pH = -log [H+]

= -log (0.334)

= 0.4763

Answer: 0.476

b)when 19.9 mL of KOH is added

Given:

M(HI) = 0.334 M

V(HI) = 39.8 mL

M(KOH) = 0.334 M

V(KOH) = 19.9 mL

mol(HI) = M(HI) * V(HI)

mol(HI) = 0.334 M * 39.8 mL = 13.2932 mmol

mol(KOH) = M(KOH) * V(KOH)

mol(KOH) = 0.334 M * 19.9 mL = 6.6466 mmol

We have:

mol(HI) = 13.29 mmol

mol(KOH) = 6.647 mmol

6.647 mmol of both will react

remaining mol of HI = 6.647 mmol

Total volume = 59.7 mL

[H+]= mol of acid remaining / volume

[H+] = 6.647 mmol/59.7 mL

= 0.1113 M

use:

pH = -log [H+]

= -log (0.1113)

= 0.9534

Answer: 0.954

c)

Since this is titration of strong acid and strong base,

the pH will be 7.00 at equivalence point

Answer: 7.00

d)when 47.8 mL of KOH is added

Given:

M(HI) = 0.334 M

V(HI) = 39.8 mL

M(KOH) = 0.334 M

V(KOH) = 47.8 mL

mol(HI) = M(HI) * V(HI)

mol(HI) = 0.334 M * 39.8 mL = 13.2932 mmol

mol(KOH) = M(KOH) * V(KOH)

mol(KOH) = 0.334 M * 47.8 mL = 15.9652 mmol

We have:

mol(HI) = 13.29 mmol

mol(KOH) = 15.97 mmol

13.29 mmol of both will react

remaining mol of KOH = 2.672 mmol

Total volume = 87.6 mL

[OH-]= mol of base remaining / volume

[OH-] = 2.672 mmol/87.6 mL

= 3.05*10^-2 M

use:

pOH = -log [OH-]

= -log (3.05*10^-2)

= 1.5157

use:

PH = 14 - pOH

= 14 - 1.5157

= 12.4843

Answer: 12.48