pKa of water. 물의 pKa 계산

pKa of water. 물의 pKa 계산

Ka of water. 물의 Ka 계산

 

 

What is the pKa of water?

 

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In most general chemistry textbooks,

the pKa of water at 25℃ is listed as 14.0.

[ Ka = 10^(-14) ]

 

In many organic chemistry textbooks, however,
the pKa of water at 25℃ is listed as 15.7.
( The value of 15.7 should not be used. )

 

 

 

 

H2O(l) + H2O(l) ⇌ H3O^+(aq) + OH^-(aq)

 

K_eq = a_H3O^+ • a_OH^- / (a_H2O)^2

≒ [H3O^+][OH^-] / (1)^2

= [H3O^+][OH^-]

 

 

 

 

또는

H2O(l) ⇌ H^+(aq) + OH^-(aq)

 

K_eq = a_H^+ • a_OH^- / a_H2O

≒ [H^+][OH^-] / (1)

= [H^+][OH^-]

 

 

 

 

Keq = 1×10^(-14) = Ka

 

 

 

[ 관련 글 https://ywpop.tistory.com/7821 ] H2O(l)의 a = 1. 평형상수의 단위는 없다

 

 

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H2O(l) ⇌ H^+(aq) + OH^-(aq)

Ka = [H^+][OH^-] / [H2O]

 

 

Kw = [H^+][OH^-] = 10^(-14) 이고,

( 참고 https://ywpop.tistory.com/2704 )

 

[H2O] = 55.56 M 이므로,

( 참고 https://ywpop.tistory.com/3076 )

 

Ka = Kw / [H2O]

= (10^(-14)) / (55.56)

= 1.7999×10^(-16)

 

 

pKa = -logKa

= -log(1.7999×10^(-16)) = 15.7448

 

 

답: pKa = 15.74

 

 

 

[참고]

pH = -log[H^+]
= -log(10^(-7)) = 7