[apstudent]

My Chem AP test is coming up and we have a review sheet to practice with. Please help with this question if you can. I have the answers listed from the teacher, i just can't figure out the work. I already figured out question (a)(bi)and(ci). I just listed them because those answers will be needed later

CaCO3(s) <--> Ca+2(aq) + CO2-(aq) Ksp = 2.8X10-9
CaSO4(s) <--> Ca+2(aq) + SO4(aq) Ksp = 9.1X10-6
The values for the solubility products for the two reactions above are given at 25 degC

a) What is the concentration of CO3-2 ions in a saturated 1L solution of CaCO3 at 25degC. (Answer = 5.29 X 10 -3)
b)Excess CaSO4(s) is placed in the solution in (a). Assume that the volume of the solution does not change.
i) What is the concentration of the SO4-2 ion? (= 3.02X10-3)
ii) What is the concentration of the CO3-2 ion?(= 9.3X10-7)
c) A .20 mole sample of CaCl2 is placed in the solution in (b). Assume that the volume of the solution does not change.
i) Whats the concentration of the SO4-2 ion(=4.6X10-5)
ii) Whats the concentration of the CO3-2 ion(=1.4X10-8)

[Astrokel]

hey!

actually bii) and cii) are the same method as ci)

well,

bii) Consider the common ion effect, what happened to the [Ca2+] in CaCO3

cii) It's exactly the same as ci)

K 

[tflint89]

I just use simultaneous eqns for these, it's laborious but it gets the job done. In b), just think that every anion (so4 2- or Co3 2-) has to come from one calcium (partly to balance charge if you like to look at it that way). The first you will need is (a+b)*a = 2.8*10-9 where a=Co3 2- and b=SO4 2- and due to the above logic, a+b has to be Ca2+. Construct a similar one for the other Ksp value and solve for a and b.

For C, similar eqns may be used but now total conc calcium has to equal sum of the 3 anions.