[shortgrl090]

i have been trying to do solubility product problems but for some reason i do not seem to be getting them right.  i attempted my homework problems but it would be nice if someone could check them and explain them if i got them wrong.  whatever someone could do to help, even if it is just a little bit, i will be grateful for. thank you so much!

Q: What is Ksp for a solution of PbF2 (subscript) if the concentration of Pb^2+ ions is 2.5 x10^-3?
A:1.25 x10^-5

Q: What is the concentration of Mg^2+ and OH^- ions when Mg(OH)2(subscript) is placed in water if the solubility product constant is 8.9x10^-12?
A: 2.11x10^-6

Q: PbCl2(subscript) is added to hot water and the solution allowed to cool.
    a. the concentration of Pb^2+ ions in solution is  0.067 M. If Ksp=1.6x10^-5, will PBCl2(subscript) precipitate out as the solution cools?
       A: yes
    b. At 25 degrees Celsius, the Ksp for PBCl2(subscript) is 1.6x10^-5. what is the [Pb^2+] and the [Cl^-]?
       A: Pb---.016   Cl----5.12x10^-4
    c. if 0.15 M KCl is added to the beaker containing lead (II) chloride in 10b, what is the new [Pb^2+]?
       A: 7.11x10^-4

[Astrokel]

hey,

it would be good if you could show us your steps to your answers as so we could point out your mistakes.

[wilson]

I quickly went through all the questions except the last one (because I don't quite understand where the 10b comes from).

Here are my answers:
1. 6.25 x 10^-8
2. Concentration of Mg²⁺=1.31 x 10^-4 ; Concentration of F^-=2.61 x 10^-4
3. [Pb²⁺][Cl^-]²=1.2 x 10^-3 ; Since this value is larger than the K_sp, hence ppt. will form.
4. Concentration of Pb²⁺=1.59 x 10^-2 ; Concentration of Cl^-= 3.18 x 10^-2

Did you take into account that 1 mole of salt can produce 2 moles of its anion in solution?
Did you remember to bring the concentrations in the expression to their respective powers depending on the molar coefficient?

Try again and see if you can get the correct answers?

Hope this helps. I hope I didn't make any mistakes though. I am human too.