Calculate the pH of a buffer prepared by mixing 300 cc of 0,3 M NH3 and 500 cc of 0,5 M NH4CI. Kb for NH3 = 1,8 × 10^-5

Calculate the pH of a buffer prepared by mixing 300 cc of 0,3 M NH3 and 500 cc of 0,5 M NH4CI. Kb for NH3 = 1,8 × 10^-5

SOLUTION

Known:

NH3 = 0,3 M

V NH3 = 300 cc

NH4Cl = 0,5 M

V NH4Cl = 500 cc

Asked: Calculate the pH of a buffer ?

Answer:

Given, kb NH3 = 1.8x10^-5

Number of moles of NH3 = MV = 0.3M x 0.3L = 0.09 M

Number of moles of N4HCl = MV = 0.5M x 0.5L

0.25 moles

kb NH3 = 1.8x10^-5

pKb = -log kb = 4.75

POH = pKb + log [salt]/[base]

POH =4.75 + log 0.25/0.09

POH = 4.75 - 0.493

POH = 4.307

pH = 14 - pOH

PH =14-4.307

Ph = 8.8117