Chemistry [Saturated solution PbCl2]

[Dousin12]

Homework Statement

To a saturated PbCl_2-solution there is a equally big volume of Pb(NO3)_2 added which has the concentration 0.05M. Is there a precipitation of PbCl2 or not? If Ks is 2,54*10^-4 for PbCl2.

The problem here is that I don't know how to find the concentration of Cl2 or Pb. My chem book said that u can solve 11g of PbCl2 per liter. So should I say that

11g/Molar mass ob PbCl2=0.04 moles <=> 0.04 moles Pb2+ <=> 0.08 Moles Cl-
But because there is 0.05M of Pb(NO3)_2 as well so the moles Pb is 0.04+0.05=0.09moles

But if i say:
[CL^-]^2*[Pb^2+]=[0.08]^2*[0.09] I get 5.76*10^-4 which is bigger than ks. But in the book they say:

[CL^-]^2*[Pb2+]=7.16*10^-5

How do they get that value?

 

[epenguin]

Homework Statement

To a saturated PbCl_2-solution there is a equally big volume of Pb(NO3)_2 added which has the concentration 0.05M. Is there a precipitation of PbCl2 or not? If Ks is 2,54*10^-4 for PbCl2.

The problem here is that I don't know how to find the concentration of Cl2 or Pb. My chem book said that u can solve 11g of PbCl2 per liter. So should I say that

11g/Molar mass ob PbCl2=0.04 moles <=> 0.04 moles Pb2+ <=> 0.08 Moles Cl-
But because there is 0.05M of Pb(NO3)_2 as well so the moles Pb is 0.04+0.05=0.09moles

But if i say:
[CL^-]^2*[Pb^2+]=[0.08]^2*[0.09] I get 5.76*10^-4 which is bigger than ks. But in the book they say:

[CL^-]^2*[Pb2+]=7.16*10^-5

How do they get that value?

The problem here more important than getting the answer is "that I don't know how to find the concentration of Cl2 or Pb". Hopefully you have got it from some other information in your book but you ought to be able to get it from the data of the problem, i.e. the solubility product, K_s.

Write the equation for K_s. Then what is the relation between the two variables that are in it? - so that we can reduce it to a single variable.

 

[Dousin12]

Im not suppose to use Ks I don't think so atleast, Because you want to know how your value is compared torwards Ks!
Because if the value is bigger than ks it will be solid and not liquid!

 

[Borek]

You can't solve this problem without Ksp. However, in the worst case, you can calculate Ksp from the solubility.

 

[Dousin12]

2,54*10^-4 I wrote in the question!

Is Ks for PbCl2

I wrote if i transform that to gram and such .. i get the values I've gotten. I see no reason why my values are wrong! But they are

 

[Borek]

First of all - do you know how to you use just given Ksp value to calculate concentrations of Pb²⁺ and Cl^- in the saturated solution?

 

[Dousin12]

Yes I am really sorry, but have u read what i wrote!

11g/Molar mass ob PbCl2=0.04 moles/dm^3 <=> 0.04 moles/dm^3 Pb2+ <=> 0.08 Moles/dm^3 Cl-'

11g is the mass of PbCl2 that is saturated! (per dm^3)

 

[Borek]

Sorry, I misread some of the things you wrote, but you mix everything at the same time and you post things that contradict itself, making your post unnecessarily hard to follow. Why do you use the solubility to calculate concentrations, if you have the Ksp? Why do you state you are not supposed to use Ksp if it is given?

One thing that you definitely did wrong is here:

But because there is 0.05M of Pb(NO3)_2 as well so the moles Pb is 0.04+0.05=0.09moles

Yes, number of moles in the mixture is sum of moles from both sources - PbCl₂ and Pb(NO₃)₂. But it doesn't mean the concentration is the sum of concentrations.

 

[epenguin]

But if i say:
[CL^-]^2*[Pb^2+]=[0.08]^2*[0.09] I get 5.76*10^-4 which is bigger than ks. But in the book they say:

[CL^-]^2*[Pb2+]=7.16*10^-5

How do they get that value?

Because they took account of the dilution or mixing factor mentioned by Borek and you didn't. (You have to do it for both ions!)

Apart from errors like that you do seem to know how to calculate an equilibrium constant from a solubility, or more exactly from the saturation concentration. It should not be too difficult then to do the inverse, which I think is what is being asked.

The homework template contains heading:

2 Relevant equations

If you had stated this, which in any case I asked in #2 your discussion would have been easier to follow, and I think you would have found the calculation easier to execute.

From what you quote it sounds like a worked example!