Question

The Iowa City Water Treatment Plant and the University of Iowa Water Treatment Plant remove calcium hardness (Ca2+) by precipitating (making) CaCO3(s). If, after establishing equilibrium with CaCO3(s), the dissolved Ca2+ concentration is 10 mg/L and the pH is 10.5.

a. What is the CO32- concentration in mol/L? b. What is the HCO3- concentration in mol/L? c. What is the alkalinity in mg/L as CaCO3?

Answer 1

a)

Given - [ Ca2 + ] = 10 mgms / L

.......................... = 10 / 20

........................ = 0.5 mlimoles / L

..................or,..... = 5.0 x 10^-4 moles / L

since , CaCO3 contains one mole of Ca2+ and one mole of CO3^2- ions , therefore the number of moles of CO3^2-

would also be equal to that of Ca^2+ = 5.0 x 10^-4 moles / L

b) Again , since one gm mole of Ca2²⁺ is equivalent to two gm. mole of HCO3^-

therefore the concentration of HCO3^2- = 5.0 x 2 x 10^-4moles / L

........................................................,or = 1.0 x 10^-3 mols / L   

c) pH of water = 10.8

[OH^- ]..........= antilog of (- 10.8 )

....................... = 1.58 x 10^-11 gm . ion moles / L

....................or,= 1.58 x 10^-11 x 17 x 1000 mgm ions / L

.........................= 2.686 x 10^-7 mgm / L

So CaCO3 equivalent in mgm / L = ( 2.686 x 10^-7 x 100) / (17 x 2 )

....................................................... = 7.9 x 10^-7 mgm / L