Chemistry buffer solution

chun hei

2012-11-17 6:12 am

A buffer solution is made by adding 5.2g of Sodium ethanoate (CH3COONa) to 1dm3 of a 0.01moldm-3 solution of ethanoic acid. Caculate the pH of the buffer solution.
(Given that molar mass of Sodium ethanoate =82gmol-1, Ka of ethanoic acid at 298K= 1.74x10-5, assume thet is novolume change on mixing)

please list all the calculation and expain detailly .

回答 (1)

賣女孩的火柴

2012-11-17 6:38 am

✔ 最佳答案

CH3COOH(aq) ⇌ CH3COO⁻(aq) + H⁺(aq) .. Ka = 1.74 x 10⁻⁵

Initially :
[CH3COOH]o = 0.01 mol dm⁻³
[CH3COO⁻]o = [CH3COONa]o= (molesof CH3COONa)/volume = (5.2/82)/1 = 0.0634 mol dm⁻³

As Ka is very small and also due to the common ion effect of thepresence of CH3COO⁻ ions in the solution, the dissociation of CH3COOH is to a verysmall extent.
Hence, at equilibrium :
[CH3COOH] ≈ [CH3COOH]o = 0.01 mol dm⁻³
[CH3COO⁻] = [CH3COO⁻]o = 0.0634mol dm⁻³

Ka = [CH3COO⁻][ H⁺]/[CH3COOH]

Take logarithm on the both sides:
logKa = log([CH3COO⁻][ H⁺]/[CH3COOH])
logKa = log[CH3COO⁻] + log[ H⁺] - log[CH3COOH]
-log[ H⁺] = -logKa + log[CH3COO⁻] - log[CH3COOH]
pH = -logKa - log([CH3COOH]/[ CH3COO⁻])
pH = -log(1.74 x 10⁻⁵) - log(0.01/0.0634)
pH = 5.56

參考: 賣女孩的火柴

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