What is the molar concentration of hydronium ions in a 0.482 M solution of formic acid, given Ka = 1.8x10^-4.  HCO2H + H2O = H30+ + HCO2-?

            H₂CO₂H(aq) + H₂O(ℓ) ⇌ H₃O⁺(aq) + HCO₂⁻   Ka = 1.8 × 10⁻⁴
Initial:      0.482 M                         0 M           0 M
Change:   -y M                            +y M          +y M
Eqm:   (0.482 - y) M                      y M           y M
              ≈ 0.482 M

At equilibrium:
Ka = [HCO₂⁻] [H₃O⁺] / [HCOOH]
1.8 × 10⁻⁴ = y² / 0.482
y = √(0.482 × 1.8 × 10⁻⁴) = 9.3 × 10⁻³
[H₃O⁺] = 9.3 × 10⁻³ M