Question

Use we keerences to access Portal values Il leeueu lui Iis question. A CH3COOH/KCH,COO buffer system is to be constructed that is also 0.041 M in Cr3+. What is the minimum [CH3COOH]/[CH3C00-] that must be maintained to keep a precipitate of chromium(III) hydroxide from forming? [CH3COOH] minimum [CH3C00]

Answer 1

minimum [CH₃COOH] / [CH₃COO^-] = 2.2 / 1

Explanation

initial concentration Cr³⁺ = 0.041 M

K_sp Cr(OH)₃ = [Cr³⁺][OH^-]³

6.7 x 10^-31 = (0.041 M) * [OH^-]³

[OH^-]³ = (6.7 x 10^-31) / (0.041 M)

[OH^-]³ = 1.63 x 10^-29

[OH^-] = (1.63 x 10^-29)^1/3

[OH^-] = 2.54 x 10^-10 M

pOH = -log[OH^-]

pOH = -log(2.54 x 10^-10 M)

pOH = 9.6

pH = 14 - pOH

pH = 14 - 9.6

pH = 4.4

According to Henderson - Hasselbalch equation,

pH = pK_a + log([conjugate base] / [weak acid])

pH = pK_a + log([CH₃COO^-] / [CH₃COOH])

4.4 = 4.74 + log([CH₃COO^-] / [CH₃COOH])

log([CH₃COO^-] / [CH₃COOH]) = 4.4 - 4.74

log([CH₃COO^-] / [CH₃COOH]) = -0.34

[CH₃COO^-] / [CH₃COOH] = 10^-0.34

[CH₃COO^-] / [CH₃COOH] = 0.457

Taking reciprocal,

[CH₃COOH] / [CH₃COO^-] = 1/0.457

[CH₃COOH] / [CH₃COO^-] = 2.189