Exam IIChemical Equlibria

LeChatlier’s PrincipleAcid DissociationBase HydrolysisTitration Curves

Solubility ProductpH Controlled Solubility

Amphoterism

Equilibria and LeChatliers Principle

1) For each of the following sets of compounds write the equilibrium reaction that would occurwhen the compounds are mixed together.

a) HNO3 and NaBenzb) NH4Cl and KOHc) NaCN and NaOH

2a) If H2 and Cl2 are added to a container, both at 2 atm, what will the pressure of HCl be afterthe system reaches equilibrium?

H2 + Cl2 ---> 2 HCl K = 150

2b) If the equilibrium pressure of HCl is 2 atm what pressures of H2 and Cl2 must have beenadded to the container originally?

3) If 100 grams of NaF and 70 grams of KOH are added to 250 mL water what is the equilibriumconcentration of all ionic species. Ka = 6.76x10-4

4) What is the pH of 100 mL of 1.2 M HAc after 20 mL of 2 M NaOH is added? Ka = 1.8x10-5

5) What is the pH of a solution that is made from 0.10 M HBenz, 0.25 M NaBenz, and 0.25 MKOH? Ka = 6.46x10-5

6) What is the pH of a solution where 100 mL of 0.50 M NH4OH is completely neutralized by 65mL of HCl? Kb = 1.8x10-5

7) Ca(IO3)2 is a slightly soluble precipitate. What would the concentration of IO3- be in a solution

saturated with Ca(IO3)2? Ksp = 1.5 x 10-15

8) What is the pH of a 0.10 M solution of NaAc? Ka = 1.8x10-5

Ac- + H2O <---> HAc + OH-

9) When ammonia is heated it decomposes to N2 and H2 according to the following reaction,

2 NH3 <----> N2 + 3 H2

Given 3 atm of NH3 and an equilibrium constant of 3x10-3, what will the final pressure (totalpressure) be in the system?

10) What is the pH of a solution made by adding 0.2 mole of NaAc to 250 mL of 1 M aceticacid? Ka = 1.8x10-5

11) In class we noted that a 0.10 M solution of calcium ions can be precipitated with 0.10 MNaOH but not in 0.10 M NH4OH. The solubility product for Ca(OH)2 is 7.88x10-6. Pleaseexplain why 0.10 M Ca2+ will not precipitate in 0.10 M NH4OH (Kb = 1.8x10-5).

12) What is the pH of the resulting solution when 0.25 mole of NaCH3CO2 and 0.15 mole HClare added to 200 mL water? The Ka for CH3COOH is 1.8x10-5M.

13) What is the pH of a solution made by adding 0.30 mole NH3(aq) to 0.50 mole NH4Cl and0.25 mole KOH? Kb for NH3(aq) = 1.8x10-5M

14) What is the concentration of all ionic and molecular species when you add 30 mL of 0.5 MNaOH to 120 mL of 0.75M HCN? Ka for HCN is 4.8x10-10.

15a) What is the pH of the following solutions when the following amounts of 1.20 M NaOHare added to 160 mL of 3 M dl Aspartic acid?

H2Asp <---> H+ + HAsp- 1.38x10-4

HAsp- <---> H+ + Asp2- 1.51x10-10

0 mL200 mL400 mL700 mL800 mL900 mL

15b) An Aspartic acid buffer of pH 5 was made by adding some NaHAsp to 0.5 moles of H2Asp.How much 1.2 M NaOH or 1.2M HCl must you add to this buffer in order to make a new bufferof pH 4.5?

16) What is the pH of the solution when 75 mL of 0.8 M dl-Histidine is titrated with thefollowing volumes of 1.20 M NaOH?

H3His <---> H+ + H2His- pKa1 = 2.40H2His- <---> H+ + HHis2- pKa2 = 6.04HHis2- <---> H+ + His3- pKa3 = 9.33

0 mL30 mL50 mL75 mL100 mL125 mL135 mL150 mL175 mL

17) How much NH4Cl and NH4OH must you add to 250 mL of water to make a buffer of pH =8.0? Kb = 1.8x10-5

18) How many moles of sodium acetate must be added to 100 mL of 0.25 M acetic acid to makea buffer of pH = 4.0? pKa = 4.74 for acetic acid.

19) Another way of making the buffer from 2 above would be to titrate the acid with a base.What volume of 0.40 M NaOH must be added to 100 mL of 0.25 M acetic acid to make a bufferof pH = 4.0? pKa = 4.74 for acetic acid.

20) How much 0.5 M NaOH must be added to 0.75 M H3PO4 to make a buffer of pH = 8? Ka1 =2.12, Ka2 = 7.21, Ka3 = 12.67

Solubility Products

21) Are the following molecules acidic, basic or neutral in aqueous solution?

NaF acid basic neutral can't tellCr(NO3)3 acid basic neutral can't tellKCl acid basic neutral can't tellNH4CN acid basic neutral can't tell

22) Are the following compounds soluble or insoluble in water?

NaIO3 Soluble Insoluble FeSO4 Soluble InsolubleCr(OH)3 Soluble Insoluble ScCl3 Soluble InsolublePbSO4 Soluble Insoluble Na2O Soluble InsolubleCaCl2 Soluble Insoluble PbSO4 Soluble Insoluble

23) What is the solubility of PbBr2 in pure water? Ksp = 4x10-5

24) What is the solubility of Ca(OH)2 in 0.05 M NaOH? Ksp = 5.5x10-6

25) What is the solubility of Ca3(PO4)2 in water? Ksp = 5.87x10-8

26) What is the solubility of Ca3(PO4)2 in 0.5 M Na3PO4? Ksp = 5.87x10-8

27) What is the solubility of AgCl in a solution of 1 M HCl? Ksp = 1.8x10-10

28) At what pH will the concentration of Cu2+ exceed 0.02 M given the following equilibrium?Cu(OH)2 ----> Cu2+ + 2 OH- Ksp = 2.2x10-20

29) How much NH3(aq) must you add to 100 grams of AgCl in order to dissolve all of the AgCl.Assume a liter of solution and calculate the concentration of NH3(aq).

AgCl ---> Ag+ + Cl- Ksp = 1.8x10-10

Ag(NH3)2+ ----> Ag+ + 2 NH3(aq) Keq = 1.6x10-9

30) Copper(I) ions in aqueous solution react with NH3 according to,

Cu+ + 2 NH3 <----> Cu(NH3)2+ Keq = 6.3x1010

Calculate the solubility of CuBr (Ksp = 5.3x10-9) in a solution in which the equilibriumconcentration of NH3 is 0.185 M.

31) What is the solubility of PbCl2 in a solution of 0.10 M H2S at a pH=0? Ksp =1.6x10-5 forPbCl2, Ksp = 8x10-28 for PbS, Keq = 1x10-20 for H2S (to 2 H+ + S2-).

Rxn = Pb2+ + H2S ---> PbS(s) + 2 H+

32) Calculate the maximum concentration of Fe3+ in an 0.10 M H2S solution buffered at pH=6.Ksp = 1.6x10-21 for Fe2S3 and Keq = 1x10-20 for H2S (to 2 H+ + S2-).

33) What is the solubility cadmium sulfide (CdS) in a saturated solution of H2S = 0.10 M at pH= 3?

CdS <--> Cd2+ + S2- Ksp = 8x10-27

H2S <--> 2H+ + S2- Keq = 1.1x10-20

34) What is the solubility (silver ion concentration) of silver sulfide (Ag2S) in a saturatedsolution of H2S = 0.10 M at pH = 3?

Ag2S <--> 2 Ag+ + S2- Ksp = 1.6x10-49

H2S <--> 2H+ + S2- Keq = 1.1x10-20

35) How much of each precipitate will form and what will the concentration of lead be if0.75 mole sodium oxalate (Na2C2O4 = Na2Ox) is added to 100 mL of solution containing 0.30 MMg(NO3)2 and 0.5 M Pb(NO3)2?

MgOx(s) <---> Mg2+ + Ox2- Ksp = 8.6x10-5

PbOx(s) <---> Pb2+ + Ox2- Ksp = 4.8x10-10

36) What is the solubility of silver carbonate (Ag2CO3) in a solution saturated with carbonic acid(0.034M) and which is buffered at pH = 9?

Ag2CO3 <---> 2 Ag+ + CO32- Ksp = 8.1x10-12

H2CO3 <--> 2 H+ + CO32- Ka = 2.02x10-17

Chemistry 121 Name_____________Second Exam April 18, 2007

CLOSED BOOK EXAM - No books or notes allowed. ALL work must be shown for fullcredit. You may use a calculator.

Question Credit

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1) Which of the following compounds is the most soluble in water? Circle one. Showyour work.

BaSO4 Ksp = 1.6x10-10

PbI2 Ksp = 1.2x10-13

Ag3PO4 Ksp = 1x10-22

2) I found a bottle marked “Acetic Acid” in our back room, but there was no indication ofconcentration. I wanted to know the concentration so I measured the pH and found it tobe pH = 2.87. What was the concentration of the acetic acid in the bottle? Ka = 1.8x10-5

3) Calculate the pH of 100 mL of 1.2 M Arsenous acid upon addition of the followingamounts of 0.8 M NaOH

H3AsO4 <---> H2AsO4- + H+ Ka1= 5.62x10-5

H2AsO4- <---> HAsO4

2- + H+ Ka2= 1.7x10-8

HAsO42- <---> AsO4

3- + H+ Ka3= 3.95x10-12

a) 0 mL

b) 150 mL

c) 225 mL

d) 400 mL

e) 450 mL

f) 600 mL

f) If you wanted to titrate to the first endpoint which indicator would you use? Circle one.

Indicator RangeMethyl Red 4.8 - 6.0Bromothymol Blue 6.2 - 7.6Cresol Red 7.0 - 8.8Phenolphthalein 8.2 - 10

g) How much 0.8 M NaOH must you add to 100 mL of 1.2 M Arsenous acid to make abuffer of pH = 11.6?

4) What is the solubility of magnesium carbonate (MgCO3) in a solution saturated withcarbonic acid (0.034M) and which is buffered at pH = 8?

MgCO3 <---> Mg2+ + CO32- Ksp = 8.1x10-12

H2CO3 <--> 2 H+ + CO32- Ka = 2.02x10-17

5) Define amphoterism. Draw a graph of pH vs. concentration as part of your example.

Chemistry 121 Name_____________Second Exam April 18, 2007

CLOSED BOOK EXAM - No books or notes allowed. ALL work must be shown for fullcredit. You may use a calculator.

Question Credit

1(10)

2(15)

3(45)

4(20)

5(10)

Total

1) Which of the following compounds is the most soluble in water? Circle one. Showyour work.

BaSO4 Ksp = 1.6x10-10

PbI2 Ksp = 1.2x10-13

Ag3PO4 Ksp = 1x10-22

2) I found a bottle marked “Acetic Acid” in our back room, but there was no indication ofconcentration. I wanted to know the concentration so I measured the pH and found it tobe pH = 2.87. What was the concentration of the acetic acid in the bottle? Ka = 1.8x10-5

3) Calculate the pH of 100 mL of 1.2 M Arsenous acid upon addition of the followingamounts of 0.8 M NaOH

H3AsO4 <---> H2AsO4- + H+ Ka1= 5.62x10-5

H2AsO4- <---> HAsO4

2- + H+ Ka2= 1.7x10-8

HAsO42- <---> AsO4

3- + H+ Ka3= 3.95x10-12

a) 0 mL

b) 150 mL

c) 225 mL

d) 400 mL

e) 450 mL

f) 600 mL

f) If you wanted to titrate to the first endpoint which indicator would you use? Circle one.

Indicator RangeMethyl Red 4.8 - 6.0Bromothymol Blue 6.2 - 7.6Cresol Red 7.0 - 8.8Phenolphthalein 8.2 - 10

g) How much 0.8 M NaOH must you add to 100 mL of 1.2 M Arsenous acid to make abuffer of pH = 11.6?

4) What is the solubility of magnesium carbonate (MgCO3) in a solution saturated withcarbonic acid (0.034M) and which is buffered at pH = 8?

MgCO3 <---> Mg2+ + CO32- Ksp = 8.1x10-12

H2CO3 <--> 2 H+ + CO32- Ka = 2.02x10-17

5) Define amphoterism. Draw a graph of pH vs. concentration as part of your example.

Amphoterism = the ability of a metal ion to be soluble in acidic and basic solutions

Concentr

atio

nofM

eta

lIon

pH0 7 14

Chemistry 121 NameSecond Exam April 10, 2008

CLOSED BOOK EXAM - No books or notes allowed. ALL work must be shown for full credit.You may use a calculator.

Question Credit

1(10)

2(15)

3(45)

4(20)

5(10)

Total

1) Which of the following compounds is the most soluble in water? Show your work.

BaF2 Ksp = 1.84x10-9

Mg3(PO4)2 Ksp = 1.04×10-15

Ag3PO4 Ksp = 1.2x10-15

2) Please write the equilibrium reaction that will occur when the following compounds aremixed. Write all reactions as dissociations where appropriate.

NaOH + HAc

Na3PO4 + HCl

HCl + KOH

3) Calculate the pH of 80 mL of 1.5 M Carbonic acid upon addition of the following amounts of0.6 M NaOH

H2CO3 <---> H+ + HCO3- Ka1= 4.3x10-7

HCO3- <---> H+ + CO3

2- Ka2= 4.7x10-11

a) 0 mL

b) 100 mL

c) 150 mL

d) 200 mL

e) 300 mL

f) 365 mL

g) 400 mL

h) 450 mL

i) How much 0.8 M NaOH must be added to 80 mL of 1.5 M H2CO3 to make a buffer of pH =10?

4) What is the solubility of Ag2S in a saturated solution of H2S = 0.01 M at pH = 3?

Ag2S <---> 2 Ag+ + S2- Ksp = 6.0x10-50

H2S <---> 2 H+ + S2- Keq = 6.84x10-23

5a) Iron can be separated from copper using pH controlled solubility in H2S. CuS is not solublein acid solutions and FeS is soluble. Draw and approximate ion separation curve for CuS andFeS.

5b) Define amphoterism and give an example.

Chemistry 121 NameSecond Exam April 10, 2008

CLOSED BOOK EXAM - No books or notes allowed. ALL work must be shown for full credit.You may use a calculator.

Question Credit

1(10)

2(15)

3(45)

4(20)

5(10)

Total

1) Which of the following compounds is the most soluble in water? Show your work.

BaF2 Ksp = 1.84x10-9 (x)(2x)2 = 4x3 = 1.84x10-9 x = 7.72x10-4

Mg3(PO4)2 Ksp = 1.04×10-15 (3x)3(2x)2 = 108x5 = 1.04x10-15 x = 3.95x10-4 most soluble

Ag3PO4 Ksp = 1.2x10-15 (3x)3 (x) = 27x4 = 1.2x10-15 x = 8.16x10-5

2) Please write the equilibrium reaction that will occur when the following compounds aremixed. Write all reactions as dissociations where appropriate.

NaOH + HAc

H2O + Ac- –> HAc + OH-

Na3PO4 + HCl

HPO42- –> H+ + PO4

3-

HCl + KOH

H2O –> H+ + OH-

3) Calculate the pH of 80 mL of 1.5 M Carbonic acid upon addition of the following amounts of0.6 M NaOH

H2CO3 <---> H+ + HCO3- Ka1= 4.3x10-7

HCO3- <---> H+ + CO3

2- Ka2= 4.7x10-11

a) 0 mL

x2/(1.5-x) = 4.3x10-7 x = 8.03x10-4M H+ pH = 3.095

b) 100 mL

½ way point so pH = 6.37

c) 150 mL - use H-H equation

(0.6M)(0.150L) = .09mole NaOH and (1.5M)(0.080L) = 0.12mole H2CO3

pH = 6.37 + log(0.09/(0.12-0.09)) = 6.847

d) 200 mL

first end point so pH = (pka1 + pka2)/2 –> pH = (6.37 + 10.33)/2 = 8.35

e) 300 mL

2nd midpoint so pH = pKa2 = 10.33

f) 365 mL - you are now 165 mL into the second region - use H-H equation

(0.06M)(0.165L) = 0.099 mole NaOH and you still have 0.12 mole of acid (HCO3-)

pH = 10.33 + log(0.099/(0.12-0.099)) = 11.00

g) 400 mL - at final endpoint pKb = Kw/Ka2 = 1x10-14/4.7x10-11 = 2.13x10-4

also [CO32-] = 0.12 mole/ (0.480L total) = 0.25M CO3

2-

x2/(0.25 - x) = 2.13x10-4 –> x = 0.007297 M OH- pH = 11.86

h) 450 mL - this is 50 mL past the final endpoint(0.050L)(0.60M) = 0.030 mole excess OH- in a total of 530 mL of solution. Therefore,[OH-] = 0.030 mole / 0.530 L = 0.0566 M OH- –> pH = 12.75

i) How much 0.8 M NaOH must be added to 80 mL of 1.5 M H2CO3 to make a buffer of pH =10? The pH is in the 2nd region so you must add 200 mL just to start this titration. Use H-H,10 = 10.33 + log(x/(0.12-x)) –> x = 0.0382 mole NaOH0.0382 mole NaOH = MV = (0.80M)(V) –> V = 0.0478 L or 47.8 mL, therefore,200 mL NaOH to get to first endpoint + 47.8 mL to get to pH = 10 = 247.8 mL of 0.80 M NaOH

M1V1 = M2V2(1.5M)(0.08L) =(0.6M)(V2)V2 = 0.20L = 200 mL to each end point

Also,pka1 = 6.37 and pka2 = 10.33

0 7 14

[Io

ns]

pH

CuS FeS

At this pH FeS is highly solubleand CuS is not very soluble.

4) What is the solubility of Ag2S in a saturated solution of H2S = 0.01 M at pH = 3?

Ag2S <---> 2 Ag+ + S2- Ksp = 6.0x10-50

H2S <---> 2 H+ + S2- Keq = 6.84x10-23

[H+]2 [S2-]/[H2S] = 6.84x10-23

[S2-] = 6.84x10-23 [H2S]/[H+]2 = 6.84x10-23 [0.01]/[1x10-3]2 = 6.84x10-19 M S2-

Substitute this result into the following equation,

[Ag+]2 [S2-] = 6x10-50

[Ag+]2 = 6x10-50/[S2-] = 6x10-50/[6.84x10-19] = 8.77x10-32

taking the square root of this answer to find [Ag+],

[Ag+] = 2.96x10-16 M Ag+

5a) Iron can be separated from copper using pH controlled solubility in H2S. CuS is not solublein acid solutions and FeS is soluble. Draw and approximate ion separation curve for CuS andFeS.

5b) Define amphoterism and give an example.Amphoterism is the ability of an ion to be soluble in either an acid or a base but not inbetween. This property is commonly exhibited with transition metals like Zinc,

Zn(OH)2(s) –> Zn2+ + 2 OH- soluble in acidZn(OH)2(s) + 2 OH- –> Zn(OH)4

2- soluble in base