求浓度为0.1mol/L的HCOOH溶液的pH值

Ka(HCOOH) = 1.77x10-4
-------------------HCOOH = H+ + HCOO-
初（mol/L) 0.1 0 0
平（mol/L) 0.1-x x x
[HCOO-] [ H+] / [HCOOH] = Ka
x^2 / (0.1 - x) = Ka
根据判断：c(HCOOH) / Ka > 500 时,可以简化：0.1 - x ≈ 0.1 代入上式求出x
x = [Ka * C] ^1/2
= (1.77x10-4 * 0.1 )^1/2
= 4.2*10^-3 = c(H+)
pH = 2.38