Posted by **Pookie** on Sunday, May 9, 2010 at 11:04pm.

What is the solubility of RaSO4 in 0.1381M Na2SO4? Ksp = 4.3x10^-11?

- Chemistry 212 -
**DrBob222**, Sunday, May 9, 2010 at 11:37pm
RaSO4 ==> Ra^+2 + SO4^-2

Ksp = (Ra^+2)(SO4^-2).

If you let S = solubility RaSO4, then

Ra^+2 = S

SO4^-2 = S from RaSO4 and 0.1381 M in Na2SO4 so a total of (S + 0.1381)

Substitute those values into Ksp expression and solve for S.

- Chemistry 212 -
**Pookie**, Sunday, May 9, 2010 at 11:44pm
Ksp = [Ra^+2][SO4^-2]

4.3x10^-11 = [x][x]

x=6.56x10^-6

6.56x10^-6 + .1381??

is that how im suppose to do it?

- Chemistry 212 -
**DrBob222**, Sunday, May 9, 2010 at 11:54pm
No.

4.3 x 10^-11 = (x)(x+0.1381)

solve for x.

NOTE:

If you solve the equation as I wrote it you will need to solve a quadratic. Those are usually easy to do with modern calculators; however, you CAN make a simplifying assumption. You make the assumption that x + 0.1381 = 0.1381 (that is, that x is so small in comparison to 0.1381 that when added it isn't different than 0.1381). The equation then would look like this.

4.3 x 10^-11 = (x)(0.1381) and solve for x. When you finish, then look at x+0.1381 and see how far it is from 0.1381. If x is so small that x+0.1381 is just 0.1381, then the assumption is ok. If not, the assumption is not ok and you must solve the quadratic. I ALWAYS make the assumption first and solve the easy equation, then check at the end and see if the assumption is ok.

- Chemistry 212 -
**Pookie**, Monday, May 10, 2010 at 12:14am
thanks a bunch!

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