A solution at 25 C has a hydronium concentration of 2.0 x 10^-6 M. What is its hydroxide concentration?

1. The two ion concentrations always multiply to 1.0 x 10^-14 at 25 degrees Celsius. 2. Rearranging gives the hydroxide concentration as that constant divided by the hydronium value. 3. Divide the coefficients: 1.0 divided by 2.0 is 0.50. 4. Subtract the exponents: 10^-14 divided by 10^-6 is 10^-8. 5. Combine: 0.50 x 10^-8 is written as 5.0 x 10^-9 mol per litre. 6. The added acid drives the autoionisation back, which is why hydroxide falls so far. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.1 Brønsted-Lowry Acids and Bases_