The acid dissociation constant ($K_a$) of acetic acid (ethanoic acid) at 298 K (25 °C) is $1.75 \times 10^{-5} \text{ mol L}^{-1}$.

• Mathematical Formula: The $\text{p}K_a$ is defined as the negative logarithm (base 10) of the dissociation constant:
  $$\text{p}K_a = -\log_{10}(K_a)$$
  $$\text{p}K_a = -\log_{10}(1.75 \times 10^{-5}) = 5 - \log_{10}(1.75) \approx 4.76$$
• Buffer Applications: An equimolar buffer solution of acetic acid and sodium acetate ($ ext{CH}_3 ext{COOH} / ext{CH}_3 ext{COONa}$) maintains a pH equal to its $\text{p}K_a$ (4.76) by the Henderson-Hasselbalch equation.
• Acid Strength: Formic acid ($\text{HCOOH}$) is stronger with a lower $\text{p}K_a$ of 3.75.
• Food Use: Acetic acid (4-8% aqueous solution) is commonly used as vinegar for food preservation and pickling.