Mass Balance and Charge Balance
Most equilibrium problems present you with analytical ("big C") concentrations of various things, then ask you to compute an equilibrium ("[ ]") concentration of some species. Two of the steps required for such solutions involve Mass Balance and Charge Balance.
Solid AgCl is added to a solution of 1.35 x 10-4 M NaCl. What is [Ag+]? Ksp AgCl = 1.82 x 10-10. Use no approximations in the solution.
First Step: Write Total Ionic Equations for everything involved.
Note that "1%" means "a very small amount" and that "99%" means most of the AgCl stays undissociated. The real amounts would have to be calculated.
We must account for all the species that make up any "big C" concentration we are given. To do this we have to "think like a chemist" and account for any and all species that will add up to "big C."
CNaCl = [Na+]
This is the only true statement we can make. CNaCl can't be equal to [Cl-], since there are two sources of Cl- in this system, the NaCl and the AgCl.
This is easy. Just look at all the ions in our total ionic equation. Don't forget to multiply each molar concentration by the charges per ion.
[Na+] + [Ag+] = [Cl-]
To solve the problem we just plug in the values