✔ 最佳答案
All numerical answers below are to 2 sig. fig.
====
Write an equation for the ionization of borate ion, H₂BO₃⁻ with water and find Kb value, and determine the pH of solution of 0.10 M sodium borate, NaH₂BO₃(aq).
H₂BO₃⁻(aq) + H₂O(l) ⇌ H₃BO₃(aq) + OH⁻(aq) ____ Kb
Kb = Kw/Ka = (1.0 × 10⁻¹⁴) / (5.4 × 10⁻¹⁰) = 1.9 × 10⁻⁵
_________ H₂BO₃⁻(aq) + H₂O(l) ⇌ H₃BO₃(aq) + OH⁻(aq) ____ Kb = (1.0 × 10⁻¹⁴) / (5.4 × 10⁻¹⁰)
Initial: _____ 0.1 M ____________ 0 M _____ 0 M
Change: ____ -y M ____________ +y M ____ +y M
At eqm: ___ (0.1 - y) M _________ y M _____ y M
As Kb is very small, then the degree of dissociation of H₂BO₃⁻ is very small.
We can assume that 0.1 ≫ y
and thus [H₂BO₃⁻] at eqm = (0.1 - y) M ≈ 0.1 M
Kb = [H₃BO₃] [OH⁻] / [H₂BO₃⁻]
(1.0 × 10⁻¹⁴) / (5.4 × 10⁻¹⁰) = y² / 0.1
y = √[0.1 ×(1.0 × 10⁻¹⁴) / (5.4 × 10⁻¹⁰)] = 1.4 × 10⁻³
pOH = -log[OH⁻] = -log(1.4 × 10⁻³) = 2.9
pH = 14.0 - pOH = 14.0 - 2.9 = 11.1
====
A borate buffer solution contains 0.15M boric acid and 0.05M sodium borate. What is the pH of this solution?
H₃BO₃(aq) + H₂O(l) ⇌ H₃O⁺(aq) + H₂BO₃⁻(aq) ____ Ka = 5.4 × 10⁻¹⁰
pH = pKa + log([H₂BO₃⁻]/[H₃BO₃]) = -log(5.4 × 10⁻¹⁰) + log(0.05/0.15) = 8.8
====
What is the molar ratio of [H₂BO₃⁻]/[H₃BO₃],in a buffer solution with pH = 8.70? If the concentration of boric acid in the buffer is 0.2 M, what should be the concentration of the borate ion?
H₃BO₃(aq) + H₂O(l) ⇌ H₃O⁺(aq) + H₂BO₃⁻(aq) ____ Ka = 5.4 × 10⁻¹⁰
pH = pKa + log([H₂BO₃⁻]/[H₃BO₃])
8.70 = -log(5.4 × 10⁻¹⁰) + log([H₂BO₃⁻]/[H₃BO₃])
log([H₂BO₃⁻]/[H₃BO₃]) = 8.70 + log(5.4 × 10⁻¹⁰)
[H₂BO₃⁻]/[H₃BO₃] = 10^{8.70 + log(5.4 × 10⁻¹⁰)}
Hence, [H₂BO₃⁻]/[H₃BO₃] = 0.27
When [H₃BO₃] = 0.2 M
[H₂BO₃⁻]/(0.2 M) = 0.27
[H₂BO₃⁻] = 0.054 M
