MF16032: What is the percent dissociation of acetic acid if the solution has pH = 4.74 and pKa = 4.74?
is it 50%
yes, 50%.
Do you know how to prove that? Did you calculate it or did you make an educated guess?
To calculate the percent dissociation of acetic acid, we need to use the Henderson-Hasselbalch equation. The Henderson-Hasselbalch equation relates the pH, pKa, and the ratio of the concentration of the dissociated and undissociated forms of an acid.
The Henderson-Hasselbalch equation is given by:
pH = pKa + log ([A-]/[HA])
In this case, acetic acid (CH3COOH) is a weak acid and it partially dissociates in water to form acetate ions (CH3COO-) and hydrogen ions (H+). The ratio [A-]/[HA] represents the extent of dissociation.
Since the pKa of acetic acid is given as 4.74, this means that at pH 4.74, exactly 50% of the acid will be dissociated and 50% will remain undissociated. This is because the log of 1 (log 1 = 0) cancels out the pKa term in the equation, leaving us with pH = pKa at the point of 50% dissociation.
Therefore, in this case, the percent dissociation of acetic acid is indeed 50%.
So, in summary, you can prove that the percent dissociation of acetic acid is 50% by using the Henderson-Hasselbalch equation and substituting the given values of pH and pKa.