Question

An ammonium buffer was prepared from 50.0ml of 0.145M NH3 and 25.0ml of 0.254M NH4Cl.

a) Write out the reactions involved in the above solution. (can be more than one)

b) what is the pH of initial buffer, NH3 Kb = 1.8 e^-5

c) what is the pH of this solution if you add 2.0ml of a strong acid, 1.1M HCl, to above buffer. Show all reactions.

Answer 1

a)

reaction involved :

NH3 (aq) +    HCl (aq) ------------------->   NH4Cl (aq)

b)

millimoles of NH3 = 50 x 0.145 = 7.25

millimoles of NH4Cl = 25 x 0.254 = 6.35

pKb = 4.74

pOH = pKa + log [salt / base]

        = 4.74 + log [6.35 / 7.25]

pOH = 4.68

pH = 9.32

c)

millimoles of acid added = C = 2 x 1.1 = 2.2

pOH = pKb + log [salt + C / base - C]

       = 4.74 + log [6.35 + 2.2 / 7.25 - 2.2]

pOH = 4.97

pH = 9.03