Question

Calculate the pH in 0.780 M CH3CO2H.

Answer 1

[CH3COOH] = 0.780 M

Solution:

Ka value of acetic acid is 1.8 E-5

Reaction of it with water and ICE

            CH₃COOH (aq) + H₂O (l) --- > CH₃COONa (aq) + H₃O⁺ (aq)

I           0.780                                                   0                      0

C         -x                                                         +x                    +x

E    (0.780-x)                                                   x                      x

Writing Ka expression

Ka = [CH₃COO^‑][H₃O⁺] / [ CH₃COOH]

Plug in the concentration at equilibrium and ka value

1.8E-5 = x²/ (0.780-x)

Since the value of ka is less than 1.0 E-4 , we make an approximation in which x in the denominator will be ignored.

Therefore,

0.780-x = 0.780

Lets substitute this value and solve for x

1.8E-5 = x²/ 0.780

X² = 1.8 E-5 x 0.780

x ² = 0.00001404

x = 0.003747

[H₃O⁺] = x = 0.003747 M

pH = -log [H₃O⁺] = - log 0.003747 = 2.43

pH of this solution would be 2.43