A saturated solution has 0.033 M carbonic acid with Ka1 = 4.3 x 10^-7. What is the hydronium concentration?

1. The two constants differ by about ten thousand, so the first step can be solved alone. 2. Let x be the hydronium formed, with the acid starting at 0.033 M. 3. The statement is 4.3 x 10^-7 = x squared over (0.033 minus x). 4. The constant is small, so take the denominator as 0.033. 5. Then x squared = 0.033 x 4.3 x 10^-7 = 1.4 x 10^-8. 6. Take the square root: x = 1.2 x 10^-4 mol per litre. 7. The bicarbonate concentration matches it, and the acid stays at 0.033 M. 8. Stopping at 1.4 x 10^-8 is the trap of quoting x squared as though it were x. _Source: OpenStax Chemistry (CC BY 4.0), Ch 14 "Acid-Base Equilibria", section 14.5 Polyprotic Acids_