Question

Calculate percent relative error in hydronium ion concentration by using concentrations instead of activities in calculating pH of a solution of the following species using thermodynamic constants.

a) 0.100 M HOAc, and 0.200 M NaOAc

b) 0.0500 M NH₃ and 0.200 M NH₄Cl

c) 0.0100 M ClCH₂COOH and 0.0600 M ClCH₃COONa.

Answer 1

(a)

Henderson-Hasselbalch equation is expressed as

pH = pKa + log { [salt] / [Acid] }

pKa of HOAc = 4.75

pH = 4.75 + log ( 0.200/0.100)

= 4.75 + log 2

= 4.75 + 0.30

= 5.05

pH = 5.05

- log [H+] = 5.05

[H+] = 10^-5.05 = 8.9 x 10^-6

(b)

Henderson-Hasselbalch equation is expressed as

pOH = pKb + log { [salt] / [Base }

pkb of ammonia = 9.25

pOH = 9.25 + log (0.200 / 0.050)

= 9.25 + log 4

= 9.25 + 0.60

= 9.85

pH + pOH = 14

pH = 14 - pOH

= 14 - 9.85

= 4.15

pH = 4.15

- log [H+] = 4.15

​ [H+] = 10^-4.15 = 7.1 x 10^-5

(c)

Henderson-Hasselbalch equation is expressed as

pH = pKa + log { [salt] / [Acid] }

pKa of HOAc = 2.85  

pH = 2.85 + log ( 0.060/0.010)

= 2.85 + log 6

= 2.85 + 0.78

= 3.63

pH = 3.63

- log [H+] = 3.63

​ [H+] = 10^-3.63 = 2.3 x 10^-4