Physical CHM4

Hey I have this question and I'm puzzled as we have never used the % before to do this.

In a 0.25M solution HY is 95% dissociated.

Caluculate the pH of this solution.

The only thing I can work out is it's a weak acid from the 95%

Reply 1

JayEm

Its a strong acid as its 95% dissociated. Any acid dissociated above 50% is considered strong.

So the pH would be -log(0.95 x0.25) = ~0.6

Robert's your father's brother.

Reply 2

mocko

dont strong acids fully dissociate in water?? I think you must be dealing with a weak acid, then again i could be wrong

Reply 3

JayEm

Taken from Wikipedia (http://en.wikipedia.org/wiki/Weak_acid):
"Acids with a Ka of greater than 55.5 are strong acids and almost totally dissociate when dissolved in water."

Note, the acid does not have to fully dissociate to be classified as strong.

Reply 4

charco

Study Forum Helper

Ka=[H+][A-]/[HA]

if Ka is 55.5 and [H+] = [A-] and for an (originally) 1 molar solution of HA then
[HA] = (1 - x) where x = [H+] = [A-]

55.5(1-x) = x^2
x^2 + 55.5x - 55.5 = 0
x= 0.986 or -56 (the negative being not possible)
so the factor x = 0.982 = [H+]
in other words it is nearly 99% dissociated

so, according to Wiki's definitions it WOULD be possible for a weak acid to be only 95% dissociated