Question

Solution C is made by mixing 58.7 mL of 0.16 M ammonium chloride (NH4Cl) with 60.1 mL of 0.26 M ammonia solution. pKa (NH₄⁺) = 9.24

a. Calculate the pH of Solution C.

b. Calculate the pH of the buffer Solution C if 0.12 g of NaOH (molar mass = 40.0 g/mol) is added (assuming no change in volume).

Answer 1

a.

pH of basic buffer = 14 - (pkb+log(NH4Cl/NH3)

                     = 14-pkb-log(NH4Cl/NH3)

    pka = 14-pkb

pH of basic buffer = pka-log(NH4Cl/NH3)

pka = 9.24

No of mol of NH4Cl = 58.7*0.16 = 9.392 mmol

No of mol of NH3 = 60.1*0.26 = 15.626 mmol

pH = 9.24 - log(9.392/15.626)

     = 9.46

b. No of mol of NaOH added = 0.12/40 = 0.003 mol

                           = 3 mmol

pH of basic buffer = pka-log((NH4Cl-NaOH)/(NH3+NaOH))

    pH = 9.24 - (log((9.392-3)/(15.626+3)))

       = 9.7