When the pH of a buffer is equal to its pKa, what is the amount of dissociation that has occurred?
1 %
25 %
50 %
100 %
50%
pH = pKa + log (base)/(acid)
So if pH = pKa then log (base)/(acid) must = 0 and (base/(acid) = 1 so (base) = (acid) so is that 50%?
Oh, you caught me at a pH question! Well, when the pH of a buffer is equal to its pKa, it means that half of the weak acid and half of the conjugate base are present. So, the answer is 50%! It's like a perfectly balanced see-saw, neither side is winning or losing. Pretty neat, huh?
When the pH of a buffer is equal to its pKa, the amount of dissociation that has occurred is 50%.
When the pH of a buffer is equal to its pKa, it means that the concentrations of the acid and its conjugate base are equal. At this point, the buffer is able to resist changes in pH when small amounts of acid or base are added. However, the amount of dissociation that has occurred depends on the initial concentrations of the acid and its conjugate base.
To determine the amount of dissociation, we need to understand the relationship between pH, pKa, and the acid dissociation constant (Ka). The pKa is defined as the negative logarithm (base 10) of the acid dissociation constant.
The formula for the acid dissociation constant (Ka) is:
Ka = [A-][H+]/[HA]
where [A-] is the concentration of the conjugate base, [H+] is the concentration of hydrogen ions (protons), and [HA] is the concentration of the acid.
When the pH of the buffer is equal to the pKa, the concentration of [H+] is equal to the concentration of [A-]. Therefore, the ratio [A-]/[HA] is equal to 1.
Let's consider an example with acetic acid (CH3COOH) and its conjugate base, acetate (CH3COO-), which have a pKa of 4.75. If we have a buffer solution where the concentration of acetic acid and acetate are 0.1 M each, we can calculate the amount of dissociation.
Using the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
When pH = pKa, the equation becomes:
pKa = pKa + log([A-]/[HA])
Taking the antilog (10^x) of both sides, we get:
1 = [A-]/[HA]
Thus, the concentrations of the acid and its conjugate base are equal, and the amount of dissociation that has occurred is 50% (half of the initial concentration of the acid has been converted to the conjugate base).