Formic acid has Ka = 1.8 x 10^-4. What is the pH of a 0.534 M solution?

1. Let x be the hydronium concentration formed, so the acid sits at 0.534 minus x. 2. The exact statement is 1.8 x 10^-4 = x squared over (0.534 minus x). 3. The acid is fairly concentrated and the constant is small, so assume x is negligible. 4. That gives x squared = 0.534 x 1.8 x 10^-4 = 9.6 x 10^-5. 5. Take the square root: x = 9.8 x 10^-3 mol per litre. 6. Check the assumption: 9.8 x 10^-3 divided by 0.534 is 1.8 percent, safely under 5 percent. 7. Take the negative logarithm: minus log of 9.8 x 10^-3 gives pH = 2.01. 8. Treating the acid as strong would give pH 0.27, and forgetting the root gives about 4.0. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.3 Relative Strengths of Acids and Bases_