KBB Gen Chem Question

[flyguy25]

I'm stuck on this problem pertaining to whether or not a solution will be saturated, unsaturated or supersaturated. It is an example problem on page 281 of the 2009-2010 KBB.

The question asks, "What are the concentrations of each of the ions in a saturated solution of PbBr2, given that the Ksp of PbBr2 is 2.1 x 10^-6? If 5 grams of PbBr2 are dissolved in water to make 1 liter solution at 25*C, would the solution be saturated, unsaturated, or supersaturated?"

I understand how to get the concentrations for both Pb and Br and getting the concentration of the solution for PbBr2 (1.36 x 10^-2 M), I just don't know what value to compare to figure out what kind of solution it is. Thanks a lot

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[JinSunah]

So I think that when you solved for the concentration of Pb2+ and Br-, the molarities you got are the concentrations in a saturated solution (8.07x10-3 M and 1.61x10-2 M). And when you compare the concentration of the solution of 5g of PbBr2 (1.36x10^-2 M), you see that the concentration is higher than the concentration of the saturated solution; therefore, it's super saturated.

Uhh that's how I think you're supposed to approach it. I remember doing the problem... but I don't remember the books explanation verbatim.. Hopefully this helps!

 

[DoCt0rPeTe]

I'm stuck on this problem pertaining to whether or not a solution will be saturated, unsaturated or supersaturated. It is an example problem on page 281 of the 2009-2010 KBB.

The question asks, "What are the concentrations of each of the ions in a saturated solution of PbBr2, given that the Ksp of PbBr2 is 2.1 x 10^-6? If 5 grams of PbBr2 are dissolved in water to make 1 liter solution at 25*C, would the solution be saturated, unsaturated, or supersaturated?"

I understand how to get the concentrations for both Pb and Br and getting the concentration of the solution for PbBr2 (1.36 x 10^-2 M), I just don't know what value to compare to figure out what kind of solution it is. Thanks a lot

I would first write out the chemical equation:

PbBr2 ------> Pb+ + 2Br-

Then write the expression for Ksp=4x^3

solve for x, the amount of moles that can be dissolved in one liter of solution. The value for Br- will be 2x, as there are twice as many moles of Br- as their are Pb+.

Now, for the second part, it is asking you to compare the value of Ksp vs. Qsp. If you don't recall what Q is, it's just the "nonequilibirum expression"

Q = 4x^3 as well.

When Q<K, your solution is not saturated.
Q=K, you are at equilibrium and a precipitate starts to form.
Q>K, a precipitate forms.

This is merely Le Chatelier's priciple...if you have a large Q (lots of products), you will shift to the LEFT and form a precipitate (the solid PbBr2 in this case).

So they give you 5g in 1 L : Calculate the number of moles using the molar mass, and then calculate your molarity.
Now plug that into
Q= 4x^3.

If Q is bigger than your K value, your precipitate will form.

I dont have a calculator or periodic table nearby, so sorry I didn't do the calculation out for you.

 

[DQLEUCD]

I'm stuck on this problem pertaining to whether or not a solution will be saturated, unsaturated or supersaturated. It is an example problem on page 281 of the 2009-2010 KBB.

The question asks, "What are the concentrations of each of the ions in a saturated solution of PbBr2, given that the Ksp of PbBr2 is 2.1 x 10^-6? If 5 grams of PbBr2 are dissolved in water to make 1 liter solution at 25*C, would the solution be saturated, unsaturated, or supersaturated?"

I understand how to get the concentrations for both Pb and Br and getting the concentration of the solution for PbBr2 (1.36 x 10^-2 M), I just don't know what value to compare to figure out what kind of solution it is. Thanks a lot

PbBr2 is insoluable so the concentrations should stay the same.

 

[DoCt0rPeTe]

PbBr2 is insoluable so the concentrations should stay the same.

Yes, according to the general qualitative solubility rules, PbBr2 would be "insoluble", in that it barely ionizes in solution.

However, this is not saying that it doesn't dissociate at all... It has a very small Ksp <<1, so it partially ionizes.

For instance, the Ksp of something really soluble like NaCl is MUCH greater than 1, so we just consider it to be qualitatively "soluble"

For quantitative problems, where values are given, they certainly want you to work out the molar solubility, which is small in this case, as the Ksp is small (as expected for a "insoluble salt")