Calculate the pH?

[匿名]

2017-11-06 12:36 pm

Calculate the pH of a 2.0 L buffer solution made by combining 0.24 M formic acid (HCOOH, Ka = 1.8x10-4) with 0.45 M sodium formate (NaHCOO).

I got 4.02 for the pH
However, I cannot understand how to finish the second part of the question.

What is the pH after 0.024 moles of HCL is added?

I tried setting up a ICE table but my answers never turn out correct.

回答 (1)

不用客氣

2017-11-06 12:49 pm

✔ 最佳答案

The first part of the question :

HCOOH(aq) ⇌ HCOO⁻(aq) + H⁺(aq) …… Ka = 1.8 × 10⁻⁴
pH = pKa + log([HCOO⁻]/[HCOOH]) = -log(1.8 × 10⁻⁴) + log(0.45/0.24) = 4.02

====
The second part of the question :

Initial n(HCOOH) = (0.24 mol/L) × (2.0 L) = 0.48 mol
Initial n(HCOO⁻) = (0.45 mol/L) × (2.0 L) = 0.9 mol
n(HCl) added = 0.024 mol

1 mole of HCl added reacts with 1 mole of HCOO⁻ ions to form 1 mole of HCOOH.
Hence, after addition HCl :
n(HCOOH) = (0.48 + 0.024) mol = 0.504 mol
n(HCOO⁻) = (0.9 - 0.024) mol = 0.876
Hence, [HCOO⁻]/[HCOOH] = n(HCOO⁻)/n(HCOOH) = 0.876/0.504

HCOOH(aq) ⇌ HCOO⁻(aq) + H⁺(aq) …… Ka = 1.8 × 10⁻⁴
pH = pKa + log([HCOO⁻]/[HCOOH]) = -log(1.8 × 10⁻⁴) + log(0.876/0.504) = 3.98

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