A soultion is prepared by mixing Mt moles of the salt MCl2 (which dissociates co

Question

A soultion is prepared by mixing Mt moles of the salt MCl2 (which dissociates completely to M^2+ +2Cl-) and Lt moles of the ligand HL in 1 L. The following reactions may occur:

M^2+ + L- <------>ML+ K=1.0x10^8                      HL(aq) <------> L- + H+ Ka= 1.0x10^-5

a) Write a mass balance for the metal species.

b) Write a mass balance for the ligand species. c) write a charge balance

d) Suppose that Mt=Lt=0.1 M (exactly) and the pH is somehow fixed at 5.00. ( This means that the charge balance no longer applies.) Use the equilibria and the two mass balances to find the concentrations of ML+, M^2+, L-, and HL.

Explanation / Answer

ANSWER:

NOTE: The question has many parts. please break it into two questions, here we answer first three parts

MCl2 <--------------> M2+ + 2Cl- -------1

M2+ + L --------> ML+   ----------2

(a) MASS BALANCE

from equation (1) rate of production of 1/2Cl- = M2+

Or Cl- = 2M2+ ------(3)

M2+exista as ML+ and M2+

therefore M2+ = ML+ + M2+

Introducing value of M2+ in equation (3)

Cl- = 2{[M2+] + [ ML+]} Mass balance equation.

(b) HL(aq) <------> L- + H+

mass balance

H+ = L- -----------(4)

L- exista as L- and ML+

Introducing value of L- in (4)

H+ = L- + ML+ Mass balance

(C) Charge balance:

n1[C1] +n2[C2] + ...... = m1[A1] +m2[A2] +....

C's are concentration of positive ions and n= charge on ion

A's are concentration of negative ions and m =charge on negative ions.

For charge balance we must include water dissociatio

H2O <------> H+ + OH-

[H+] + 2[M2+] + [ML+] = [OH-] + [Cl-] + [L-]

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