In a buffer solution, if [Aƒ{] < [HA], which of the following must be true?
a. pH < pKa
b. pH = pKa
c. pH > pKa
d. pH < 7.00
e. pH > 7.00
less than
greater than
To determine which statement must be true when [A-] is less than [HA] in a buffer solution, we need to consider the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the concentration of the acid and its conjugate base:
pH = pKa + log([A-]/[HA])
Here, pKa represents the acid dissociation constant and [A-] and [HA] represent the concentrations of the conjugate base and acid, respectively.
Given that [A-] < [HA], we can rewrite the Henderson-Hasselbalch equation as:
pH = pKa + log([A-]/[HA]) < pKa + log(1) = pKa
From this, we can conclude that the statement "pH < pKa" must be true. Therefore, the correct answer is option a: pH < pKa.
Here is the Henderson-Hasselbalch equation.
pH = pKa + log [(base)/(acid)].
base is A
acid is HA
If base < acid, the fraction must be less than 1, the log of a number less than 1 is negative; therefore, pH must be .....than pKa.