Question

Tap water at room temperature has a pH of 7.2 and carbonate alkalinity of 200 mg/L (= 40 mmol/L). What is the concentration of bicarbonate and carbonate in mol/L?

Answer 1

Answer:

the concentration of bicarbonate is [HCO₃⁻] = 0,03996 M and carbonate is [CO₃²⁻] = 3,56x10⁻⁵ M.

Explanation:

Carbonate-bicarbonate is:

HCO₃⁻ ⇄ CO₃²⁻ + H⁺ With pka = 10,25

Using Henderson-Hasselbalach formula:

pH = pka + log₁₀$\frac{[CO_{3}^{2-}]}{[HCO_{3}^-]}$

7,2 = 10,25 + log₁₀$\frac{[CO_{3}^{2-}]}{[HCO_{3}^-]}$

8,91x10⁻⁴ = $\frac{[CO_{3}^{2-}]}{[HCO_{3}^-]}$ (1)

Also:

0,040 M = [CO₃²⁻] + [HCO₃⁻] (2)

Replacing (2) in 1:

[HCO₃⁻] = 0,03996 M

Thus:

[CO₃²⁻] = 3,56x10⁻⁵ M

I hope it helps.