Question

A water sample has an alkalinity of 135 mg/L as CaCO₃ and a pH of 8.8. Determine the concentrations [mg/L] of HCO₃^-, CO₃^-2 and OH^-.

Answer 1

CaCO₃ + H₂O + CO₂ Ca(HCO₃)₂

We have,

Molecular weight of CaCO₃= 100 g/mol

Molecular weight of HCO₃^- =61 g/mol

Each mole of CaCO₃ reacts to 1 mole Ca(HCO₃)₂

Now, 100 g of CaCO₃ reacts to 2 x 61 = 122 g of HCO₃^-

1 g of CaCO₃ reacts to = (122/100) g of HCO₃^-

a) Therefore, alkalinity due to HCO₃^- = 135 x (122/100) = 164.7 mg/L

b) Alkalinity due to CO₃²-= 0.6 x 135 = 81 mg/L

c) Here, 34 g OH^- are equivalent to 100 g CaCO₃

Therefore, concentration of OH^- = (34/100) x 135 = 45.9 mg/L