Answer:
Ka = 9.2x10⁻⁷
Explanation:
The equilibrium of carbonic acid in water is:
H₂CO₃ ⇄ HCO₃⁻ + H⁺
Where Ka is defined as:
Ka = [HCO₃⁻] [H⁺] / [H₂CO₃]
The equilibrium concentration of the species is:
[H₂CO₃] = 1.9x10⁻² - X
[HCO₃⁻] = X
[H⁺] = X
As pH is -log[H⁺]
3.88 = -log[H⁺]
1.318x10⁻⁴ = [H⁺] = X
Replacing:
[H₂CO₃] = 1.9x10⁻² - 1.318x10⁻⁴ = 1.8868x10⁻²
[HCO₃⁻] = 1.318x10⁻⁴
[H⁺] = 1.318x10⁻⁴
Replacing in ka equation:
Ka = [1.318x10⁻⁴] [1.318x10⁻⁴] / [1.8868x10⁻²]
