Here’s the experiment we did to determine the Ksp of PbCl!
1. Take a 300 mL beaker from a cabinet, wash it thoroughly, and dry it with a paper towel. This is to get rid of any chemical residue that might alter this experiment.
2. Place the 300 mL beaker on top of an electric balance. Zero out the mass and then ACCURATELY measure 1.46 g of “powder” sodium chloride (common table salt) by pouring very slowly with a scupula.
3. Take the beaker off the balance and then add 250 mL of water into the beaker using a graduated cylinder to transfer the water into the beaker with the sodium chloride with the best accuracy. (Mathematically, you would end up with 0.100 M NaCl)
4. Leave the unstirred solution alone and then shift your focus to the lead (II) nitrate.
5. Repeat Step 1.
6. Repeat Step 2, except ACCURATELY pour 6.37 g of the “solid” lead (II) nitrate by using the scupula again. (Try to minimize skin contact with this substance for it could cause potential harm)
7. Repeat Step 3 using the lead (II) nitrate. (Mathematically, you would end up with 0.100 M Pb(NO3)2)
8. Now stir both the sodium chloride solution and the lead (II) nitrate by using a stirring rod to agitate the solute so that it will dissolve more efficiently. Once the particles are dispersed throughout the whole solvent, take out a 600 mL beaker and proceed to pour in the NaCl solution. Then, pour in the lead (II) nitrate solution.
9. Continue to stir until you notice a white substance forming on the bottom of the beaker. Filter the substance out using filter paper, funnel, and an Erlenmeyer flask. Then retrieve the white substance and using a scupula and move it to a new, clean beaker (any size bigger than 200 mL is fine). This is the precipitate lead (II) chloride.
10. Then measure 250ml of tap water into another beaker and place in the solid lead (II) chloride. This procedure will allow you to determine the solubility product constant (Ksp) of this supposedly insoluble solid.
This experiment illustrates the two most important concepts of Ksp: precipitation and determination of the solubility product constant. Precipitation was shown by mixing two soluble substances (sodium chloride and lead (II) nitrate) after mixing each respectively in water. If this reaction was written in a chemical equation, it would be: 2NaCl + Pb(NO3)2 –> 2NaNO3 + PbCl2 (s). Two ionic compounds formed an insoluble solid using the double-reaction method and according to the solubility rules, PbCl2 is an exception for all Cl– being soluble.
After the precipitate is formed, we can determine the value of the solubility product constant by using a reference, preferably a source listed in the main page. The Ksp value for lead (II) chloride happens to be 1.7 x 10-5. In order to figure out the constant, which we designate by the variable X, we need to set up the Ksp equation: 1.7 x 10-5 = [Pb2+] x [Cl–]2. [Pb2+] = X and [Cl– ] = 2X because of mole ratios between ions. Then plug back into the equation to get: 1.7 x 10-5 = [X] x [2X]2. Using algebra, X = 1.6 x 10-2 M, which is the solubility product constant. This is how much an insoluble solid would dissolve in water. So this whole experiment was to show how you can derive the constant by creating a precipitate and then dissolving it in water to find Ksp.