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In water, carbonic acid has ionization constants K1 = 4.2 × 10⁻⁷ and K2 = 4.8 × 10⁻¹¹. For a saturated 0.034 M solution of carbonic acid, choose the correct statement.

1. The concentration of CO3²− is 0.034 M.
2. The concentration of CO3²− exceeds that of HCO3−.
3. The concentrations of H+ and HCO3− are nearly the same.
4. The concentration of H+ is twice the concentration of CO3²−.

Correct answer: The concentrations of H+ and HCO3− are nearly the same.

Solution

Carbonic acid ionizes mainly in its first step (K1 >> K2), giving roughly equal [H+] and [HCO3-]. [CO3^2-] ~ K2 is tiny, and the second ionization is negligible, so the concentrations of H+ and HCO3- are nearly the same.

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