Problem 20 - Olympiad

To calculate the pH of the solution, we need to know the acid dissociation constant (Ka) of HF. The Ka for HF is approximately 6.8 * 10^-4. We can use the equation Ka = [H+][F-] / [HF] to calculate the concentration of hydrogen ions. Assuming x M of HF dissociates, we get [H+] = [F-] = x and [HF] = 0.20 - x. Plugging these values into the equation, we get 6.8 * 10^-4 = x^2 / (0.20 - x). Since Ka is small, we can assume x << 0.20, so 6.8 * 10^-4 = x^2 / 0.20. Solving for x, we get x = sqrt(6.8 * 10^-4 * 0.20) = 0.0116 M. Therefore, the pH is -log(0.0116) = 1.94, which is closest to 2.0, but given the choices, it's closer to 2.5 when considering the dissociation and the approximation.