To calculate the pH of a 0.2 M solution of acetic acid, we can use the Ka expression and the formula for pH. Ka = [H+][A-]/[HA]Where Ka is the acid dissociation constant, [H+] is the concentration of hydrogen ions, [A-] is the concentration of acetate ions, and [HA] is the concentration of acetic acid.Let x be the concentration of H+ ions and A- ions that dissociate from the acetic acid. Then, the concentration of acetic acid remaining is 0.2 - x .Now, we can set up the Ka expression:1.8 x 10^-5 = x x / 0.2 - x Since Ka is very small, we can assume that x is much smaller than 0.2, so we can simplify the equation to:1.8 x 10^-5 = x^2/0.2Now, solve for x:x^2 = 1.8 x 10^-5 * 0.2x^2 = 3.6 x 10^-6x = 3.6 x 10^-6 x 1.9 x 10^-3Now that we have the concentration of H+ ions x , we can calculate the pH using the formula:pH = -log[H+]pH = -log 1.9 x 10^-3 pH 2.72So, the pH of the 0.2 M solution of acetic acid at 25C is approximately 2.72.