Question

Calculate the alkalinity as CaCO₃ of water that contains 85 mg/L of HCO₃^-, 120 mg/L of CO₃^2-, and 2 mg/L of OH^-. (10 points)

Answer 1

Step 1: calculate the molar mass of HCO₃^- , CO₃^2- AND OH^-

HCO₃^-= 61 g/mole

CO₃^2-=60 g/mole

OH^- =17 g/mole

step 2: determine the equivalent weight (Eq.) of each component

Equivalent weight is obatined by dividing the molar mass by pxidation number or ionic charge

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

Eq. weight CO₃^2- = 60 / 2 (charge) = 30 g/Eq.
Eq. weight OH^- = 17/1(charge) = 17 g/Eq.

step 3: determination of number of eqivalents

number of equivalents is calculated by dividing masses of individual components by their equivalent weight respectively

Number of Eq. (HCO₃-) =85/61 =1.3934 Eq.

Number of Eq.( CO₃^2-)= 120/30=4 Eq.

Number of Eq. (OH^-) =2/17=0.117647 Eq.

Step 4: Calculation of alkinity as CaCO3

For alkalinity of CaCO₃ we will consider only HCO₃- and CO₃^2- . OH will not be considered

Number of Eq. (CaCO3) = 1.3934 Eq + 4Eq =5.3934 Eq/L