# Dr.Bob (thx) Check my work for this one pls

What is the molar solubility of Sr3(AsO4)2 and the concentration of Sr^2+ and AsO4^-3 in a saturated solution at 25 deg Celsius? The Ksp is 1.3E-18. Show the balanced reaction.
Below, I write M.S. (molecular solubility)

1)Sr3(AsO4)2<---> 3Sr^2+ + 2AsO4^-3

M.S. xmol/L 3x 2x

(3x)^3 (2x)^2 = 108x^5
(kindly explain why it is (3x^3)(2x)^2 as I followed an diff. example from the book)
Solving for x I get 1.04E-4

So the molar solubility: 1.04E-4 mol Sr3(AsO4)2/L (dissolved); then the concentrations of the ions are:
[Sr^2+] = 3x = 3.1E-4
[AsO4^3-] = 2x = 2.1E-4

If these are correct than your help is starting to sink in, if not than a little more practice should do. By the way, I don't mind lenghty detailed replies, it only shows that you are as passionate about the subject as I'd like to be. You must be a chemist! Thx again Dr.Bob Also, I imagine that anyone is free to reply to a post; I have replied to some chemistry and math posts, but I am not sure if I'm breaking any rules. It is a fun website and I do learn. Six week course, particularly chem II is rough. Way too fast. :)We're coverring
"Eclectro chemistry, metals II and coordination compounds". That make three chapters in two more days before dreaded finals next monday! Phew! Wish me luck!

Your work is impeccable. I THINK you will understand the (3X)^3(2x)^2 business if you had written the Ksp expression. For this reaction it is
Ksp = (Sr^+2)^3(AsO4^-3)^2.
(Sr^+2) = 3X and that is cubed in the Ksp expression so it becomes (3X)^3.

(AsO4^-3) = 2X and that is squared in the Ksp expression to become (2X)^2
and voila!, (3X)^3(2X)^2 = etc.

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