CHM 2046C Module 12      Name: ____ Sample Answers___

 

Part N:  Predicting Precipitation Reactions                  10 points

 

If the concentration of the strontium ion, Sr 2+, is 2.5 x 10 -4 M, does precipitation of SrSO4 occur when enough of the soluble salt  Na2SO4 is added to make the solution,  2.5 x 10 -4 M? (Ksp for SrSO4 is 3.4 x 10 -7).

 

 (b) If no precipitate forms, what must the concentration of the Sodium Sulfate be in order for precipitation to begin?

 

Use the experimental ion concentrations to calculate the reaction quotient, Q. Then compare the Q and the Ksp to decide if the system is at equilibrium (when Q = Ksp).

 

  1. If Q = Ksp, the solution is saturated.
  2. If Q < Ksp the solution is not saturated and no precipitate will form.
  3. If Q > Ksp, the system is not at equilibrium; the solution is supersaturate and SrSO4 will precipitate until Q = Ksp

 

For Strontium Sulfate:

 

SrSO4   (s) ↔    Sr2+ (aq)  +   SO42- (aq) 

 

For Sodium Sulfate ( a very soluble salt-all Sodium salts are soluble):

 

Na2SO4   (s) →    2Na1+ (aq)  +   SO42- (aq)

 

The Sodium ion is a spectator and is unreactive, while the concentration of the sulfate ion is 2.5 x 10 -4 M and provides the Sulfate common Ion, so that 

 

Q = [Sr2+] [SO42-] = [2.5 x 10-4][ 2.5 x 10-4 ] = 6.25 x 10-8

 

The Ksp for Strontium Sulfate is 3.4 x 10 -7

Therefore the Ksp > Q  or 3.4 x 10 -7  > 6.25 x 10-8  via Case #2 above the solution is not saturdated and NO precipitate will form

 

(b) If no precipitate forms, what must the concentration of the Sodium Sulfate be in order for precipitation to begin?

 

Ksp = [Sr2+] [SO42-]

 

3.4   x 10 -7  = [2.5 x 10-4][ SO42-]

 

when [ SO42-] = 1.36 x10-3 M makes Q = Ksp and creates a saturated solution, Any addition amount of Sodium Sulfate added, which cause precipitation of Strontium Sulfate.

 

Reference McMurry Section 15.13 pages 629-631 which develops the concepts in Sections 5.14 and 15.15 and Qualitative Analysis. See worked example 15.14 page 630. Try problems 15.19 and 15.30 on pages 630-631. Also try probelms 15.130-15.134 page 642.

 

Additional worked example from another textbook: