酸鹼計算相關

HC2H3O2之Ka = 1 x 10^-5

1.
取0.1M的[HC2H3O2]的水溶夜20mL與0.3M，30 mL的[HC6H5O2]混合，HC6H5O2的Ka = 2 x 10^-5，則此水溶液的pH值為何？

HC2H3O2(aq) ⇌ H^+(aq) +C2H3O2^-(aq)
HC6H5O2(aq) ⇌ H^+(aq) +C6H5O2^-(aq)

只考慮稀釋效應，混合後：
[HC2H3O2]o = 0.1 x [20/(20 + 30)] =0.04 M
[HC6H5O3]o = 0.3 x [30/(20 + 30)] =0.18 M

平衡時，假設有 a M 的 HC2H3O2和 b M 的 HC6H5O2解離。
並設0.04 M >> a M 及0.18 M >> bM。

平衡濃度：
[HC2H3O2] = (0.04 - a) M ≈0.04 M
[C2H3O2^-] = a M
[HC6H5O2] = (0.18 - b) M = 0.18 M
[C6H5O2^-] = b M
[H^+] = (a + b) M

Ka(HC2H3O2):
a(a + b)/0.04 = 1 x 10^-5 ...... (1)

Ka(HC6H5O2):
b(a + b)/0.18 = 2 x 10^-5 ...... (2)

(2)/(1):
0.04b/0.18a = 2
b = 9a ...... (3)

(3)代入(1):
a(a + 9a)/0.04 = 1 x 10^-5
a = 0.0002

a + b = 0.0002 + 9x0.0002
[H^+] = (a + b) M = 0.002 M
pH = -log(0.002) = 2.7


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2.若AgC2H3O2之Ksp = 3.6 x10^-3，將AgC2H3O2加入水中進行水解反應，最終水溶液的pH= 4，則AgC2H3O2的溶解度為？

溶解：AgC2H3O2(s)⇌Ag^+(aq) + C2H3O2^-(aq)
水解：C2H3O2^-(aq)+ H2O(l)⇌ HC2H3O2(aq)+ OH^-(aq)

由於水解產生OH^-(aq) 離子，故溶液應呈鹼性，但題目給出酸性的 pH4，故為矛盾，此題無解。