Exam 3 -- sample problems in the area of equilibrium

 

These are typical of the equilibrium problems that could show up on exam 3.

 

I will not provide solutions, but I provide a section devoted to deciphering the question so it fits on of the standard methods of equilibrium problems.

 

The breakdown (solving) of the problems should be available by Tuesday morning.

 

Type 1 -- A mixture is prepared that is probably not at equilibrium. In which direction will the reaction mixture shift?

 

1. I mix equal volumes of 0.10 M AgNO3 and 0.050 M Na2SO4.

            Will a precipitate of Ag2SO4 be produced?

 

2.  A sample of  MxSO4 . 2 H2O and MxSO4 is exposed to air that has a relative humidity of 75% at 23oC. Will the sample gain or lose weight?

            Kp =  510    atm-2     for the reaction   MxSO4 + 2 H2O(g) ----> MxSO4 . 2 H2O

            The equilibrium vapor pressure of water is 0.0355 atm.  at 23oC.

 

3. A drop of acid base indicator , H-In, is placed in a buffer with pH = 6.40

            K(indicator)  =  2.0 x 10-5 = [H+][In-, blue] / [HIn, red]

            what color is the new solution ?

 

4. The equilibrium constant for the reaction  2NO2 (g) ---> N2O4 (g) is 105. atm-1

            A mixture is at a pressure of 1.55 atm and it is 17% NO2 and 83% N2O4. When it reaches equilibrium (a) will the P(NO2) increase or decrease?  (b) will the total pressure increase, decrease or stay about the same ?

 

 

Type 2 -- A mixture is prepared as indicated. What is the equilibrium mixture line?

 

5. A weak acid HA has pKa of 6.47 ; what is the pH of a solution that is 0.15 M in HA ?

6. What’s the pH of a 0.050 M solution of Na A  (the sodium salt of that acid)

7. How much AgCl precipitates when we add concentrated HCl (to a total of 0.050M) to 2.0 liters of a 0.025M Solution of AgNO3. (We can assume the volume stays nearly 2.0 liters)

8. What is the pH at the midpoint when we titrate _____ acid with NaOH ?

9. The Ksp of Cr(OH)3 is ____. How many moles of Cr(OH)3 will dissolve in 1.0 liters of water that maintains a pH of 9.50 ?

10 A weak acid is 0.25% dissociated when the cncentration is 0.10 M in HA. What is Ka ?

 

 

Type 3 -- Here’s the properties of an equilibrium mixtue; what is K ?

 

9. The solubility of  Mx(OH)2 is 0.00037 moles per liter. What is Ksp of the compound.

10. A 0.050 M solution of the waek acid, HA, has pH of 2.45 ; what is Ka ?

 

11. We start with a 0.0020 M solution of Mx2+ ion.  We add ammonia until the solution has [NH3] = 1.50  M.  We have an electrode that measures [Cu2+] and it says the concentration of Cu2+ is 2.7 x 10-9 M.  What is Kstability  for the reaction

            Mx2+ + 4 NH3 ---> [Mx(NH3)4]2+

 

Type 4 -- converting K’s

12. An acid has pKa = 2.55; what is Ka for the acid and Kb for the conjugate base?

 

13. Ksp = 2.55 x 1-12 for the reaction MxCl2 (s) --> Mx2+  + 2Cl-

            what’s Keq  for the reaction Mx2+  + 2Cl-   -->  MxCl2 (s)

 

14. What’s the [H3O+] conc and pH of a solution that’s 0.0010 M in Ba(OH)2, a strong base.

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15. titration-- A weak acid HA has Ka = 4.5 x 10-4

            what the pH as we titrate a 0.10M solition of HA with 0.50 M NaOH?

            assume we start with 100 ml of the weak acid.

                        what is the pH after adding ... 0 ml, 5 ml, 10 ml, 15 ml, 20 ml and 25 ml of base

                        (an exam is likely to only ask about one of these)

                        when is the pH of solution equal to pKa for this acid.