Question

Calculate the pH of each of the following solutions containing two solutes in the concentrations indicated.

            (a)        0.50 M HOAc (acetic acid), 0.5 M NaOAc (sodium acetate); K_a = 1.8 x 10^-5

            (e)        0.400 M NaHSO4 , 0.400 M Na2SO4 ; K_a = 1.2 x 10^-2

Answer 1

Equation to calculate pH of a buffer - Henderson Hasselbalch equation:
pH = pKa + log ([A-] /[HA]
Where [A-] = molar concentartion of conjugate base - [CH3COO-] = 0.5M
[HA] Molar concentration of weak acid - [CH3COOH] = 0.5M
pKa = -logKa = -log 1.8*10^-5 = 4.74
Substituting:
pH = 4.74 + log( [0.5]/[0.5])
pH = 4.74+log1.0
pH = 4.74+0
pH = 4.74

e)Na2SO4=base = 0.400 M

NaHSO4=conjugate acid = 0.400 M

pH = pKa + log ([A-] /[HA]

pKa = -log 1.2 x 10^-2 = 1.92

pH = 1.92 + log [conjugate acid]/ [base]

pH = 1.92 + log( 0.400/ 0.400)

= 1.92