A weak acid (represented as HA) dissociates in water to produce a hydronium ion
ID: 883944 • Letter: A
Question
A weak acid (represented as HA) dissociates in water to produce a hydronium ion (represented as H+ ) and the conjugate base (represented by A ).
The acid dissociation constant, Ka, for the weak acid is defined by the equation:
Ka =
If you rearranged the above equation and too the logarithm, you would obtain the Henderson-Hasselbalch equation:
pH = pKa + log
The Henderson-Hasselbalch equation is used to calculate the pH of a solution that contains both a weak acid and its conjugate base. Such a solution is called a buffer, which means that the pH of the buffer is rather insensitive to addition of a strong acid or base when the pH is near the pKa value. Suppose that you have a solution that has concentrations of acetic acid and sodium acetate equaling 0.7, and 0.3respectively. If the pKa value of acetic acid is 4.72, what is the pH of the solution
[H3O+ ] . [A- ] [HA]Explanation / Answer
Using the Hendersen-Hasselbalck equation,
pH = pKa + log[A-]/[HA]
and,
feeding the given values,
[A-] = 0.3
[HA] = 0.7
pKa = 4.72
we have,
pH = 4.72 + log(0.3/0.7) = 4.35 of the buffer