Nitrous acid is HNO2. Its dissociation is HNO2 = H(+) + NO2(-). The equilibrium constant, Ka = [H+][NO2-]/[HNO2] => x^2 / (0.20 - x). Given that it is a weak acid (Ka = 4.6 * 10^ -4),you can use a very good approxiamation: 0.20 >> x => 0.20 - x = 0.20, and this permits you to solve the value of x more easily. => 4.6 * 10^-4 = (x^2) / 0.20 => x^2 = 0.20 * 4.6 * 10^ -4 = 0.92 * 10^-4 => x = 9.59 * 10^ -3. Now,calculate pH = log { 1 / [H+] } = log { 1 / (9.59 * 10 ^-3) } = 2.02. Answer pH = 2.02
