# Solubility Equilibria

### In-class Problem Set #5

Calculate the solubility of lead(II)phosphate under the following constraints.

a) No other simultaneous equilibria will occur.

What reaction is occurring and what is the equilibrium constant expression?

Students should recognize that lead(II)phosphate will dissociate in water, but may have trouble remembering what the stoichiometric coefficients are in front of each ion. Allow the students a few minutes to write the correct reaction and Ksp expression. Once everyone has the correct expression, spend a few minutes discussing sparingly soluble complexes, the solubility product, saturated solutions, and solubility.

What two expressions equate the individual dissociated ions to the solubility of the solid?

If students are struggling with this step, ask them something along the lines of:

If I measured the concentration of free lead(II), what would that tell me about how many moles of the solid dissolved?

Students may try to incorporate Ksp­­ into their expression or try to account for both ions in the same expression. Remind them that there is one expression for each ion. It may take about ten minutes for every student to have and comprehend the right expressions.

What is the value of S?

Students will probably not be able to answer this question on their own. Remind them of the two expressions they just wrote for S and that they also have a Ksp expression for the solution.  Groups will eventually realize that the two expressions for S can be rearranged and substituted into the Ksp expression leaving S as the only variable.

Using the value for S, what is the concentration of free lead(II) in this system?

Again, the students will not necessarily know how to approach this questions, but remind them that they have an expression which relates S to the concentration of each ion.