- We have a buffer solution since the weak base(ammonia) and it's salt or conjugate acid(ammonium chloride) are present in the solution.
- The pH is easily calculated using Handerson equation:
pKa could be calculated from given pKb as:
Pka = 14 - pKb
Pka = 14 - 4.74 = 9.26
let's plug in the values in the equation to calculate the original pH of the buffer:
pH = 9.26 + 0
pH = 9.26
Now, 0.05 moles of NaOH are added. It's a strong base so it would react with the acid(ammonium ion) present in the buffer as:
(sodium and chloride ions are not included here as they are the spectator ions and so they get canceled out.)
- ammonium ion and hyroxide ion(NaOH) reacts in 1:1 mol ratio to give ammonia(base).
- So, 0.05 moles of hydroxide ion will react with exactly 0.05 moles of ammonium ion to form 0.05 moles of ammonia.
pH = 9.26 + 0.48
pH = 9.74
Change in pH
= 9.74 - 9.26
= 0.48