A 0.0516 M solution of nitrous acid has pH 2.34. What is Ka for the acid?

1. Convert the pH: the hydronium molarity is ten to the power minus 2.34, which is 0.0046 M. 2. Each ionisation event makes one hydronium ion and one nitrite ion together. 3. So the nitrite concentration at equilibrium is also 0.0046 M. 4. The acid lost that same amount: 0.0516 minus 0.0046 = 0.0470 M. 5. Here the change is not negligible, so subtracting it properly matters. 6. Numerator: 0.0046 x 0.0046 = 2.12 x 10^-5. 7. Divide by 0.0470 to get 4.5 x 10^-4 as the acid ionisation constant. 8. Dividing by the original 0.0516 instead of the equilibrium value shifts the answer noticeably. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.3 Relative Strengths of Acids and Bases_