1. The problem statement, all variables and given/known data A particular sample of vinegar has a pH of 2.90. If acetic acid is the only acid that vinegar contains (Ka = 1.8 x 10-5), calculate the concentration of acetic acid in the vinegar. 2. Relevant equations Ka = [Products]/[Reactants] pH = -log[H+] 3. The attempt at a solution I arranged the problem in my usual lazy way: Acid + Water ---> Conjugate Base + Hydrogen Ions Assuming that [H+] is equal to [Conjugate Base] I calculated the concentration of the conjugate base and hydrogen ions. pH = -log[H+] = 2.90 [H+] = 10-2.90 = [Conjugate Base] Then I plugged those numbers and the given Ka value into: Ka = [products]/[reactants] 1.8 x 10-5 = [10-2.90]2/[Acid] [10-2.90]2/1.8 x 10-5 = [Acid] I get 8.8 x 10-12. The book says the answer is .089 M. Not quite sure where I'm going wrong.. maybe I am not using the correct equations or perhaps it's an overlooked error in algebra. Help!