Overall, the solubility of the reaction decreases with the added sodium chloride. This therefore shift the reaction left towards equilibrium, causing precipitation and lowering the current solubility of the reaction. The reaction quotient for PbCl 2 is greater than the equilibrium constant because of the added Cl. To simplify the reaction, it can be assumed that is approximately 0.1M since the formation of the chloride ion from the dissociation of lead chloride is so small. The equilibrium constant remains the same because of the increased concentration of the chloride ion. Notice that the molarity of Pb 2 + is lower when NaCl is added. For example, a solution containing sodium chloride and potassium chloride will have the following relationship: Contributions from all salts must be included in the calculation of concentration of the common ion. In calculations like this, you can always assume that the concentration of the common ion is entirely due to the other solution. If the salts contain a common cation or anion, these salts contribute to the concentration of the common ion. Even fairly dilute solutions of the OH- ion have more than enough OH- ion to precipitate Cu(OH)2 from an 0.10 M Cu2+ ion solution. If several salts are present in a system, they all ionize in the solution. The solubility products K sp's are equilibrium constants in hetergeneous equilibria (i.e., between two different phases).
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