CHM 2046C  Module 12 Chapter 18 Name: __ Answers__

 

I. Estimating Salt Solubility from Ksp       10 points

 

Given the following insoluble salts, estimate the salt’s molar solubility from the Ksp :

 

1.   Cu(OH)2  (s)   +   HOH  (l)    ß à  Cu2+ (aq)     +  2 OH1- (aq)

 

Ksp = 2.2 x 10 -20

 

Ksp = [Cu2+][OH1-]2

 

If x = amount (moles)  of Copper II hydroxide that dissolves in a liter of water, then

 

2.2 x 10 -20    = [x][2x]2  = 4x3

 

x3 = 5.5 x 10 -21    

 

Therefore the molar solubility from Ksp is

x =  1.7  x 10 -7    mol/L

 

 

 

2.   MgCO3  (s)   +   HOH  (l)    ß à    Mg2+ (aq)   +    CO32- (aq)

 

Ksp = [Mg2+][ CO3 2-]                                                    Ksp = 6.8 x 10 -6

 

If x = amount (moles)  of Magnesium carbonate that dissolves in a liter of water, then

 

6.8 x 10 -6= [x][x]  = x2

 

x2 = 6.8 x 10 -6    

 

Therefore the molar solubility from Ksp is

x =  2.6  x 10 -3    mol/L

 

 

 

3.   Al2(SO4)3  (s)   +   HOH  (l)    ß à  2 Al3+ (aq)   +  3  SO42- (aq)

 

Ksp = 9.8 x 10 -21

 

Ksp = [Al3+]2[ SO42-]3                                                    

 

If x = amount (moles)  of Aluminum sulfate that dissolves in a liter of water, then

 

9.8 x 10 -21= [2x]2[3x]3  = 108x5

 

x5    = 9.1 x 10 -23  = 910 x 10 -25 

 

Therefore the molar solubility from Ksp is

x =  3.9  x 10 -5   mol/L

 

 

 

 

 

4.   Cu2S  (s)   +   HOH  (l)      ß à 2 Cu1+ (aq)   +    S2- (aq)

 

Ksp = 2.2 x 10 -48

 

Ksp =    [Cu1+][S2-]                                                                 Ksp = 2.2 x 10 -48

 

If x = amount (moles)  of Copper I sulfide that dissolves in a liter of water, then

 

2.2 x 10 -48    = [2x]2 [x]  = 4x3

 

x3 = 550 x 10 -51    

 

Therefore the molar solubility from Ksp is

x =  8.2  x 10 -17    mol/L

 

McMurry Reference:

Read Section 15.11 pages 619-622. Study Worked Example 15.10 page 521. Do Problem 15.23 on page 521.

Try Problems 15.96 and 15.97 page 642 at the end of the chapter

 

Kotz Reference:

See Section 18.4 Relating Solubility and Ksp pages 875-878.  Review worked examples 18.9 and 18.10 pages 876-877. Work Exercise 18.13 page 878. At the end of the chapter there are eight additional exercises:  #45-#52 pages 896-7.

 

 

 

 

Additional Worked Example: