Question

某未知浓度的一元弱酸用未知浓度的NaOH 滴定，当用去3.26 mL NaOH 时，混合溶液的pH=4.00 ，当用去18.30 m

某未知浓度的一元弱酸用未知浓度的NaOH 滴定，当用去3.26 mL NaOH 时，混合溶液的pH=4.00 ，当用去18.30 mL NaOH时，混合溶液的pH=5.00 ，求该弱酸的电离常数。

Answer 1

设未滴定前，酸碱浓度相等，并设酸的体积为Va
pH＝pKa + lg(cs/ca)＝pKa+lg(ns/na)＝pKa+lg[VNaOH/( Va－VNaOH)]
即：4＝ pKa +lg[3.26/( Va－3.26)]
5＝ pKa +lg[18.3/( Va－18.3)]
解方程组得：pKa ＝5.02 Ka＝8.5×10－6

追问

首先，谢谢您的回答。但为什么要设酸碱浓度相等呢？

追答

这样可以才好求出答案

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