CH302H - Principles of Chemistry II: Honors
Spring
2014, Unique 51880

Homework, Week 10


Homework, week 10
Answer Key

Homework week 10  (Solubility and complex formation)

In some cases, there are “hints” at the bottom of the problem set.  In these cases the word Hint is shown after the question.  Try struggling with the question for a while before jumping to the hint which, by the way, is often not a “big hint”.

 

1.   a)  What is the molar solubility of BaSO4(s) in water?  Ksp = 4.3x10-11 (ignore hydrolysis of SO4 to form HSO4-.)

b)  Would this solid be more soluble in an acidic or basic solution?  Why or why not? Hint

c)   OK, let’s make this a bit more challenging interesting… Give a reasonable estimate of the molar solubility of BaSO4(s) in a solution that is buffered at pH= 2  (For H2SO4  Ka1 = Big;  Ka2=1.03x10-2). 

 

2.   Ni(OH)2(s) is minimally soluble in water.  (Ksp = 6x10-16)  To what pH would you have to buffer the solution in order to bring [Ni2+] to 1.00x10-3 M?

3.   6.0 g of NaCl is added slowly to a 1.0 L solution containing both 1x10-3 M Ag+ and Pb2+.  (For AgCl Ksp = 1.8x10-10; for PbCl2 Ksp = 1.7x10-5)

a)  What will precipitate first as the NaCl is added?

b)  After all the NaCl is added and the system reaches equilibrium, what are the concentrations of all solution species? Hint

 

4.   a)  It seems peculiar that the solubility of PbF2 increases if HNO3 is added to the solution, but the addition of this acid to PbCl2 has little effect.  Why might that be? Hint

b) You are initially puzzled and figure that you perhaps need a different acid for the PbCl2(s) so you add HCl.  Yikes!  Now, even less PbCl2 dissolves than it did in plain water!  What is going on!!!

 

5.  One form of a chemical heating pad consists of a solution of supersaturated sodium acetate (CH3CO2Na).  When a “physical shock” (a small clicker disk) is given the solution it proceeds to equilibrium, precipitates sodium acetate and generates heat.

a)  From this information, is the heat of solution for sodium acetate greater than or less than 0?

b)  After the pack has reached equilibrium and you wanted to try to rejuvenate it by once again making a supersaturated solution.  Would you heat or cool the solution in an attempt to redissolve the solid?

c)  A cold pack can be made by dissolving solid ammonium nitrate in water.  In this case, is the heat of solution for ammonium nitrate greater than or less than 0?

 

6.   Copper tetraamine (AKA tetraaminecopper(II)) is a water-soluble complex formed between copper and the ammonia ligand, Cu(NH3)42+).  It has step-wise formation constants of 1x104, 2x103, 5x102 and 9x101 for K1 to K4, respectively.  Kf  = K1 K2 K3 K4 = 9x1011.   

a)  Given a solution that is 1.0 F in NH3 and 1x10-3 F in Cu2+, calculate the concentration of all copper containing species in the solution.  (Ignore the acid/base character of NH3)  Hint

b)  Let’s assume that we have 1.0 g of the relatively insoluble solid, CuCO3(s), sitting at the bottom of a 1 L beaker of water.  (For CuCO3(s)  Ksp = 2.3x10-10, i.e., not very soluble… to say the least.)  I intend to dissolve this by forming the soluble Cu(NH3)2+ complex by adding NH3 to the solution.  (We’re going to neglect any volume change for this problem.)  What is [NH3] in the solution once I get all of the CuCO3(s) to dissolve?  (Let’s only worry about Kf and assume that only the Cu(NH3)42+ complex forms.)   Hint

 

7.   “Austin's alkaline soil can help get the lead out of homegrown veggies” is the title of an Austin American Statesman article back in 2009 (http://testmygarden.com/austin-tx-soil-testing_5_1322066470.pdf) 

a)  Can you explain chemically why this seems like a reasonable statement… or, at least, “seem logical”  that we should have less concern than those places with more acidic soils?  Hint

b)  If you lowered the pH of the soil by one pH unit, how much would you increase the mobile [Pb2+] ? 

 

8.  Scale (CaCO3(s)) precipitates from water, forms on pipes and can severely restrict (or stop!) the flow of water.  It is a particular problem in hot water pipes, hot water heaters, boilers and the like both in your home and in industry.  Is  DH  >0, =0 or <0 for the reaction   Ca2+(aq) + CO32-(aq) à CaCO3(s)?  

Any idea of how you might mitigate this problem?

Is this type of temperature dependence that you might typically expect when thinking of the dissolution (or precipitation) of a solid? 

 

= = = = =  Spoiler alert:  Don’t look at hints below until you tried the above on your own = = = = = =

 

 

 

 

 

 

 

 






Hints for selected problems:

 

1b  Consider using the exact treatment remembering that you cannot write a charge balance expression because you are in a buffer.  Is there anything unique about the solution composition because pH ~ pKa2?  Remember:  In a system at equilibrium, all equilibrium constants must be satisfied.

3b  Be sure to see the final concentrations are not more than you started with!

4    Think LeChatlier and the impact (or lack of impact) by added acid. 

6    a)  Think LeChatlier to consider which species dominates, i.e., there is a lot of NH3 present relative to the Cu2+, yes?  Are the Ks for the formation of the complexes large or small? 
b)  Step at at time… if all the solid disappears, what is the CO32- concentration in solution? … and what would free [Cu2+] have to be?  Can you guess at how much of the copper from the original CuCO3(s) must be tied up in the soluble amine complex?  All set now?

7    You have to visualize the “soil” as a solution of a given pH with Pb(OH)2(s) sitting at the bottom of the beaker.  This is not a bad visualization since the plants feed from the water in the soil; and now we simply have to assume that that water and its solutes are in equilibrium with the soil composition.