We performed a lab in class about a month ago concerning different types of equilibrium. One particular portion of the lab adding 5 mL of .3M HCl to 5 mL of .3 M Pb(NO3)2. After a precipitate finally formed, 8 mL of water was added to dissolve PbCl2.

Question

One of my lab questions asks, "Explain why PbCl2 did not precipitate immediately on addition of HCl. (What condition must be met by [Pb+2] and [Cl-] if PbCl2 is to form?

I'm stumped on this question. By multiplying the concentrations of Pb+2 and Cl- like so:
(.3)x(.3)^2
I get a value of .027, which is higher than most Ksp values I've seen for lead chloride. Why, then, would the precipiation of the solid not be immediate upon the addition of HCl?

I don't know. What I wonder about is the effect of the acid pH on the ksp. This was not neutral water.

PbCl2 forms at least a couple of complex ions with HCl. One is PbCl3^- and the other more common one is PbCl4^-2. Usually, however, the PbCl2 will form first before some of it dissolves by forming the complexes.

Your question doesn't provide much about the manner in which the HCl was added to the Pb(NO2)2. There is the possibility, however, that the HCl was added in small increments and the PbCl2 didn't ppt right away because Ksp was not exceeded right away. The question about "what must be so about Pb and Cl" almost begs the answer "[Pb^+2][Cl^-]^2 must be greater than Ksp before pptn will take place.

Answer 1

the pbcl2 had not the right concentration level

Answer 2

I think that your concentrations of Pb and Cl are lower than you think they are. If you were to add 1L of 1M Pb+ to 1L of 1M Cl-, remember that you now have doubled the amount of liquid. You now have 2 L of liquid with 1 mole of Pb+ and 1 mole of Cl- in it. As you add your Cl-,the value gets squared, so, you will eventually get to a point where it puts you over your needed Ksp.

Answer 3

123