A 0.125 M solution of nitrous acid has pH 2.09. What is its percent ionisation?

1. Convert the pH back to a concentration: the hydronium molarity is ten to the minus 2.09. 2. That works out at 8.1 x 10^-3 mol per litre. 3. Each nitrous acid molecule that ionises gives one hydronium ion, so this is the amount ionised. 4. Form the ratio: 8.1 x 10^-3 divided by 0.125 = 0.065. 5. Multiply by 100 to get 6.5 percent. 6. Quoting 0.65 percent is a slipped decimal point from the same division. 7. The logarithm carried two significant figures, so the answer is quoted to two as well. 8. Inverting the ratio would give about 15, which is not a percentage of anything meaningful. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.3 Relative Strengths of Acids and Bases_