Hardness is defined as the sum of the divalent cations in water (cations have a

Question

Hardness is defined as the sum of the divalent cations in water (cations have a positive ionic charge), expressed as CaCO3.Typically the major cations that contribute to hardness are Ca^2+ and Mg^2+. A water supply was analyzed and it was found to have 180 mg/L of Ca^2+ and 24.3 mg/L of Mg^2+. What is the hardness?(Hint: convert mass concentrations to equivalent concentrations.)

A water supply is analyzed for carbonate species. The pH of the sample is 7.0 and the concentration of HCO3- is 22.5 eq/L. What is the alkalinity of the water as (a) eq/L and b) as CaCO3? (Hint: Do you need to include the equivalents of CO3^2- in calculating alkalinity for this sample? Also, remember the meaning of p notation, it will help you calculate theH^= and OH^- concentrations.)

Explanation / Answer

molar mass of HCO3- = 61gm/mol

Molar mass (CO3(2-)) = 60 g/mole.
Molar mass (CaCO3) = 100 g/mole.

weight(HCO3(-)) = 61 / 1 (charge) = 61 g/Eq

weightCO3(2-)) = 60 / 2 (charge) = 30 g/Eq

weight(CaCO3) = 100 / 2 (oxidation state) = 50 g/Eq.

Number of Eq. (HCO3(-)) = 0.35g / 61 g/Eq = 0.0057 Eq.
Number of Eq. (CO3(2-)) = 0.12g / 30 g/Eq = 0.004 Eq.

Number of Eq. (CaCO3) = 0.0057 Eq + 0.004 Eq = 0.0097 Eq/L.

Alkalinity as CaCO3 = 0.0097 Eq/L x 50 g/Eq = 0.485 g/L = 485 mg/L.

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