Question

CO2 dissolves in water to form carbonic acid: H2CO3 <---> H+ + HCO3-Ka1 = 4.3 x l0^-7 HCO3- <---> H+ + CO3-2 Ka2 = 4.8 x l0^-11 In an acid solution with pH =4.35 ([H+ ]=4.5x10^-5), what is the ratio [CO3-2] /[H2CO3]?

10^-2

10^-4

10^-6

10^-8

Answer 1

Ratio of [CO₃²⁻] / [H₂CO₃] is 1.0x10⁻⁸

Step-by-step explanation:

Based on the equilibriums:

H₂CO₃ ⇄ H⁺ + HCO₃⁻ Ka1 = 4.3x10⁻⁷

HCO₃⁻ ⇄ H⁺ + CO₃²⁻ Ka2 = 4.8x10⁻¹¹

The sum of both equilibriums is:

H₂CO₃ ⇄ 2 H⁺ + CO₃²⁻ K' = 4.3x10⁻⁷*4.8x10⁻¹¹ = 2.064x10⁻¹⁷

Where K' is defined as:

K' = 2.064x10⁻¹⁷ = [H⁺]² [CO₃²⁻] / [H₂CO₃]

If concentration of H⁺ is 4.5x10⁻⁵M:

K' = 2.064x10⁻¹⁷ = [4.5x10⁻⁵]² [CO₃²⁻] / [H₂CO₃]

1.0x10⁻⁸ = [CO₃²⁻] / [H₂CO₃]

Thus, ratio of [CO₃²⁻] / [H₂CO₃] is 1.0x10⁻⁸