Question

a) What is the final proudct of polyphosphonate hydrolysis?

b) HCl in natural water can react with solid CaCO₃ and increase the hardness and alkalinity in water. What reaction shows how HCl increases the hardness of water and its alkalinity? If 1 mM HCl is present as an anthropogenic contaminant in water with solid CaCO₃, what is the hardness of water after equilibration?

Answer 1

a) The end product of Hydrolysis of Polyphosphonates typically depends on the extent of Hydrolysis and lenght of the polyphosophonate chain.

Typically the end products are Ortho phosphates (PO4) if the hydrolysis is complete or it shall be condensed (smaller) polyphosphates if it is a long Polyphosphonate chain,.

Part B - Explaination

Part C Calculation

What is the hardness of Water after Equilibration:

To answer this part of question all the chemical equations that exist in equilibrium are to be solved:

1. Ca²⁺_(aq) + CO₃^2-_(aq) <-> CaCO_3(s)

2. CO₃^2-_(aq) + H⁺_(aq) <-> HCO^-_3aq) + H⁺_(aq) <-> H₂CO_3(aq) <-> H₂O + CO_{2 (g)}

For simplicity we ignore the equilibrium between H₂CO₃ (Aqueous) and CO₂ (Gaseous phase) and the second equilibrium between HCO₃^-_(aq) and H₂CO_3(aq) as the k_a (10^-10.25) for the diprotic acid is very low.

If in the equilibrium concentration of CO₃^2- is x and Concentration of HCO3- y

Concentration of Ca2+ in Solution = x+y

From chemical equation 2 y = 10^-3 moles (Total amount of bicarbonate produced = Total amount of acid consumed).

Ionic Product of CaCO3 in solution = x * (x+y)

                                                          = x * (10^-3+x)

Since the solution shall be saturated at equilibrium

Ionic Product = Solubility product (Ksp)

Ksp of CaCO3 = 6 X 10 ^-9

• x^2 + 10^-3 * x = 6*10^-9
• Simplifying the equation x = 6* 10^-6

So total Calcium in the solution = x+y = 10^-3+ 6*10^-6

                                                            = 10^ -3 mol/lit(Approx.)

            Hardness = 40 x 10^-3 *10^3 mg/lit as Cas

                        = 40 gm/lit as Ca

                        = 40/20*50 gm/lit as CaCO3

Hardness       = 100 ppm as CaCO3