5 Steps to a 5 AP Chemistry 2019

370 ❯ STEP 5. Build Your Test-Taking Confidence

(ii) From part (i), the second ion (Sr^2 +) begins to precipitate at 3.2 × 10 -^6 M SO 42 -. Using this value

with the BaSO 4 Ksp allows the calculation of the barium ion concentration when the SrSO 4 first

begins to precipitate. This calculation is:

[Ba^2 +] = (^) −
[SO]
sp
4
2

K

= 1.1 10

3.2 10

10

6

×

×

−

−^ = 3.4 ×^10

• (^5) M Ba 2 +
  You get 1 point for this answer. You also get 1 point if you if you did this calculation correctly but
  made the wrong prediction in part (i).
  (c) For the compounds with the same stoichiometry (BaSO 4 , CaSO 4 , and SrSO 4 ) the one with the largest
  Ksp is the most soluble; therefore, it is only necessary to compare the solubilities of Ag 2 SO 4 and CaSO 4.
  For CaSO 4
  Ksp = [Ca^2 +] [SO 42 - ] = 9.1 × 10 -^6 and [Ca^2 +] = [SO 42 - ] = x
  Ksp = x^2 = 9.1 × 10 -^6 so x = 3.0 × 10 -^3 M
  For Ag 2 SO 4
  Ksp = [Ag+] [SO 42 - ] = 1.6 × 10 -^5 and [Ag+] = 2 [SO 42 - ], [Ag+] = 2 x and [SO 42 - ] = x
  Ksp = (2x)^2 (x) = 4 x^3 = 1.6 × 10 -^5 so x = 1.6 × 10 -^2 M
  The value for x from Ag 2 SO 4 is larger; therefore it is more soluble.
  Give yourself 1 point for this answer.
  There is a total of 4 points possible. Subtract 1 point if any answer has the wrong number of significant
  figures.
  21-Moore_PE02_p341-370.indd 370 31/05/18 1:54 pm