What is the pH of 0.100 M acetic acid before any base is added, given Ka = 1.8 x 10^-5?

1. Acetic acid is weak, so the hydronium concentration must come from its constant. 2. Assume the amount that dissociates is small against 0.100 M. 3. Then the hydronium concentration squared equals the constant times 0.100. 4. Multiply: 1.8 x 10^-5 x 0.100 = 1.8 x 10^-6. 5. Take the square root: 1.3 x 10^-3 mol per litre. 6. Take the negative logarithm: pH = 2.87. 7. Treating the acid as strong would give pH 1.00, which is the first distractor. 8. The value 4.74 belongs to the half equivalence point, not to the starting solution. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.7 Acid-Base Titrations_