Which of the following equations do not hold true at all temperatures (may be more than one correct answer)?
A) pH = -log[H+]
B) 14.00 = pH + pOH
C) pKa = pH + pOH
D) kw = [H+][OH-]
E) [OH] = 10^-pOH
Please explain why the answer is correct
pH is defined as the negative logarithm of molar concentration of [H]+. pH = -log[H]+
Hence option (a) is true always.
The dissociation constant of water is 1×10˄-14 at 25ᵒC and 1 atmospheric pressure. Therefore, Kw = [H]+×[OH]- = 1×10˄-14,
Taking negative logarithm of both sides,
(-log[H]+) + (-log[OH]-) = 14 (at 25ᵒC)
Or, pH + pOH = 14
Just as temperature variation alters any equilibrium, at any other temperature, this equilibrium is also likely to be altered and the equilibrium constant value shifted.
So, option (b) is not true at any temperature; it is very much temperature dependent.
pKa refers to the acid dissociation constant of any weak acid, it has nothing to do with [H]+.[OH]-
Hence option (c) is never true.
(d) Water dissociates as H2O = H+ + OH-,
kw = [H+][OH-], this is true always by definition of the terms.
(e) pOH = -log[OH]-
`rArr` [OH]- = 10˄-pOH
This is true at all temperatures.