Question

How to create a buffer solution with pH 8.5?

- The buffer solution should have a volume of 100 mL
- It must have the capacity to maintain its natural pH with in +/- 0.1 units on the addition of either 1 mL of 1 M HNO3 or 1 mL of 1 M NaOH
- Give the chemical formulas of the buffer constituents, the chemical equation of the buffer equilibrium and the pKb of the equilibrium

Answer 1

Chemical formulas of the buffer constituents: NH₄OH/NH₄Cl

NH_3(aq) + H₂O_(l) NH₄⁺(aq) + OH^-_(aq)

Assuming:

Since Kb is small and it's present the common ion NH₄⁺, the concentration of NH₃ at equilibrium is essentially equal to its initial concentration.

So,

pOH = 14 - pH = 14 - 8.5 = 5.5

Add 1 mL of 1M HNO₃:

NH_3(aq) + H₃O⁺_(l) NH₄⁺(aq) + OH^-_(aq)

0.1 0.001 0.5696     (Initial)

0.099 0 0.5706 (Final)

Add 1 mL of 1M NaOH:

NH_3(aq) + H₂O_(l) NH₄⁺(aq) + OH^-_(aq)

0.1 0.5696 0.001 (Initial)

0.101 0.5686 (Final)