Answer:
To prepare 100 mL of a 0,050M sodium citrate tribasic buffer to a pH of 6 you need to add 7,16 mL of 0,5M HCl, 1,4705 g of sodium citrate tribasic dihydrate and complete 100 mL with water.
Explanation:
The acid equilibrium of sodium citrate tribasic buffer is:
citrate dibasic⁻² ⇄ citrate tribasic⁻³ + H⁺ pka = 6,4
Using Henderson-Hasselbalch formula:
pH = pka + log₁₀ [tex]\frac{[A^{-}]}{[HA]}[/tex]
6,0 = 6,4 + log₁₀ [tex]\frac{[CitrateTribasic]}{[CitrateDibasic]}[/tex]
0,3981 = [tex]\frac{[CitrateTribasic]}{[CitrateDibasic]}[/tex] (1)
You need to add 0,1L× 0,050M = 0,0050moles of sodium citrate:
0,0050 moles = Citrate tribasic + Citrate dibasic (2)
Replacing (2) in (1)
Citrate dibasic: 3,58x10⁻³ moles
Thus,
Citrate tribasic: 1,42x10⁻³ moles
Citrate tribasics reacts with HCl thus:
Citrate tribasic⁻³ + HCl → Citrate dibasic⁻² + Cl⁻
Thus, you need to add 5x10⁻³moles of sodium citrate tribasic and 3,58x10⁻³ moles of HCl:
5x10⁻³moles of sodium citrate tribasic×[tex]\frac{294,1 g}{1mole}[/tex] = 1,4705 g of sodium citrate tribasic dihydrate -commercial reactant-
3,58x10⁻³ moles of HCl÷ 0,5 M = 7,16x10⁻³ L ≡ 7,16 mL of 0,5M HCl
Thus, to prepare 100 mL of a 0,050M sodium citrate tribasic buffer to a pH of 6 you need to add 7,16 mL of 0,5M HCl, 1,4705 g of sodium citrate tribasic dihydrate and complete 100 mL with water.
