EQUILIBRIUM QUESTIONS
Year
2011
2010
2009
2008
2007
2006
2005
2004
2003
2002
2001
2000
1999
1998
1997
1996
1995
1994
1993
1992
1991
1990
1989
1988
1987
1986
1985
1984
1983
1982
1981
Version A
Acid/Base (Ka)
Solubility (Ksp)
Acid/Base (Ka)
Gas (Kp/Kc)
Acid/Base (Ka)
Solubility (Ksp)
Acid/Base (Ka)
Solubility (Ksp)
Acid/Base (Kb)
Acid/Base (Ka)
Solubility (Ksp)
Gas (Kp/Kc)
Acid/Base (Kb)
Solubility (Ksp)
Acid/Base (Ka)
Acid/Base (Ka)
Gas (Kp/Kc)
Solubility (Ksp)
Acid/Base (Ka)
Gas (Kp/Kc)
Acid/Base/Buffer (Ka)
Solubility (Ksp)
Acid/Base (Ka)
Gas (Kp/Kc)
Acid/Base/Precipitation (Ka/Ksp)
Acid/Base (Ka)
Solubility (Ksp)
Acid/Base (Ka)
Gas (Kp/Kc)
Acid/Base/Buffer (Ka)
Gas (Kp/Kc)
Version B
Acid/Base/Solubility (Ka,Kb,Ksp)
Gas (Kc)
Acid/Base (Kb)
Gas (Kp/Kc)
Gas (Kp/Kc)
Acid/Base (Ka)
Acid/Base (Ka)
Gas (Kp/Kc)
Gas (Kp/Kc)
2011 A
1.
Each of three beakers contains 25.0 mL of a 0.100 M solution of HCl, NH3, or NH4Cl, as shown above. Each
solution is at 25°C.
(a) Determine the pH of the solution in beaker 1. Justify your answer.
(b) In beaker 2, the reaction NH3(aq) + H2O(l)  NH4+(aq) + OH–(aq) occurs. The value of Kb for NH3(aq) is 1.8
10-5 at 25°C.
(i)
(ii)
Write the Kb expression for the reaction of NH3(aq) with H2O(l).
Calculate the [OH–] in the solution in beaker 2.
(c) In beaker 3, the reaction NH4+(aq) + H2O(l)  NH3(aq) + H3O+(aq) occurs.
(i)
(ii)
Calculate the value of Ka for NH4+(aq) at 25°C.
The contents of beaker 2 are poured into beaker 3 and the resulting solution is stirred. Assume that
volumes are additive. Calculate the pH of the resulting solution.
(d) The contents of beaker 1 are poured into the solution made in part (c)(ii). The resulting solution is stirred.
Assume that volumes are additive.
(i)
(ii)
Is the resulting solution an effective buffer? Justify your answer.
Calculate the final [NH4+] in the resulting solution at 25°C.
2011 A form B
1.
Answer the following questions about the solubility and reactions of the ionic compounds M(OH)2 and
MCO3, where M represents an unidentified metal.
(a) Identify the charge of the M ion in the ionic compounds above.
(b) At 25°C, a saturated solution of M(OH)2 has a pH of 9.15.
(i) Calculate the molar concentration of OH–(aq) in the saturated solution.
(ii) Write the solubility-product constant expression for M(OH)2.
(iii) Calculate the value of the solubility-product constant, Ksp for M(OH)2 at 25°C.
(c) For the metal carbonate, MCO3, the value of the solubility-product constant, Ksp is 7.4×10-14 at 25°C.
On the basis of this information and your results in part (b), which compound, M(OH)2 or MCO3, has the
greater molar solubility in water at 25°C? Justify your answer with a calculation.
(d) MCO3 decomposes at high temperatures, as shown by the reaction represented below.
MCO3(s)  MO(s) + CO2(g)
A sample of MCO3 is placed in a previously evacuated container, heated to 423 K, and allowed to come to
equilibrium. Some solid MCO3 remains in the container. The value of Kp for the reaction at 423K is 0.0012.
(i) Write the equilibrium-constant expression for Kp of the reaction.
(ii) Determine the pressure, in atm, of CO2(g) in the container at equilibrium at 423 K.
(iii) Indicate whether the value of ∆G° for the reaction at 423 K is positive, negative, or zero. Justify your
answer.
2010A
1. Several reactions are carried out using AgBr, a cream-colored silver salt for which the value of the
solubilityproduct
constant, Ksp , is 5.0 × 10−13 at 298 K.
(a) Write the expression for the solubility-product constant, Ksp , of AgBr.
(b) Calculate the value of [Ag+] in 50.0 mL of a saturated solution of AgBr at 298 K.
(c) A 50.0 mL sample of distilled water is added to the solution described in part (b), which is in a beaker with
some solid AgBr at the bottom. The solution is stirred and equilibrium is reestablished. Some solid AgBr
remains in the beaker. Is the value of [Ag+] greater than, less than, or equal to the value you calculated in
part (b) ? Justify your answer.
(d) Calculate the minimum volume of distilled water, in liters, necessary to completely dissolve a 5.0 g sample
of AgBr(s) at 298 K. (The molar mass of AgBr is 188 g mol-1.)
(e) A student mixes 10.0 mL of 1.5 × 10−4 M AgNO3 with 2.0 mL of 5.0 × 10−4 M NaBr and stirs the resulting
mixture. What will the student observe? Justify your answer with calculations.
(f) The color of another salt of silver, AgI(s), is yellow. A student adds a solution of NaI to a test tube
containing a small amount of solid, cream-colored AgBr. After stirring the contents of the test tube, the
student observes that the solid in the test tube changes color from cream to yellow.
(i) Write the chemical equation for the reaction that occurred in the test tube.
(ii) Which salt has the greater value of Ksp : AgBr or AgI ? Justify your answer.
2009 A
1.
Answer the following questions that relate to the chemistry of halogen oxoacids.
(a) Use the information the table below to answer part (a)(i).
Acid
Ka at 298 K
HOCl
2.9  10-8
HOBr
2.4  10-9
(i) Which of the two acids is stronger, HOCl or HOBr? Justify your answer in terms of Ka.
(ii) Draw a complete Lewis electron-dot diagram for the acid that you identified in part (a)(i).
(iii) Hypoiodous acid has the formula HOI. Predict whether HOI is a stronger acid or a weaker acid than
the acid that you identified in part (a)(i). Justify your prediction in terms of chemical bonding.
(b) Write the equation for the reaction that occurs between hypochlorous acid and water.
(c) A 1.2 M NaOCl solution is prepared by dissolving solid NaOCl in distilled water at 298 K. The hydrolysis
reaction OCl-(aq) + H2O(l)  HOCl(aq) + OH-(aq) occurs.
Write the equilibrium-constant expression for the hydrolysis reaction that occurs between OCl-(aq)
and H2O(l).
(ii) Calculate the value of the equilibrium constant at 298 K for the hydrolysis reaction.
(iii) Calculate the value of [OH-] in the 1.2 M NaOCl solution at 298 K.
(i)
(d) A buffer solution is prepared by dissolving some solid NaOCl in a solution of HOCl at 298 K. The pH of the
buffer solution is determined to be 6.48.
(i)
(ii)
Calculate the value of [H3O+] in the buffer solution.
Indicate which of HOCl(aq) or OCl-(aq) is present at the higher concentration in the buffer solution.
Support your answer with a calculation.
2008 A
C(s) + CO2 (g)  2 CO(g)
1.
Solid carbon and carbon dioxide gas at 1,160 K were placed in a rigid 2.00 L container, and the reaction
represented above occurred. As the reaction proceeded, the total pressure in the container was
monitored. When equilibrium was reached, there was still some C(s) remaining in the container. Results
are recorded in the table below.
Time
(hours)
Total Pressure of Gases
in Container at 1,160 K
(atm)
0.0
5.00
2.0
6.26
4.0
7.09
6.0
7.75
8.0
8.37
10.0
8.37
(a) Write the expression for the equilibrium constant, Kp for the reaction.
(b) Calculate the number of moles of CO2(g) initially placed in the container. (Assume that the volume of the
solid carbon is negligible.)
(c) For the reaction mixture at equilibrium at 1,160 K, the partial pressure of the CO2(g) is 1.63 atm. Calculate
(i)
(ii)
the partial pressure of CO(g), and
the value of the equilibrium constant, Kp
(d) If a suitable solid catalyst were placed in the reaction vessel, would the final total pressure of the gases at
equilibrium be greater than, less than, or equal to the final total pressure of the gases at equilibrium
without the catalyst? Justify your answer. (Assume that the volume of the solid catalyst is negligible.)
In another experiment involving the same reaction, a rigid 2.00 L container initially contains 10.0 g of C(s), plus
CO(g) and CO2(g), each at a partial pressure of 2.00 atm at 1,160 K.
(e) Predict whether the partial pressure of CO2(g) will increase, decrease, or remain the same as this system
approaches equilibrium. Justify your prediction with a calculation.
2007 A
HF(aq) + H2O(l)  H3O+(aq) + F–(aq)
Ka = 7.210–4
1. Hydrofluoric acid, HF(aq), dissociates in water as represented by the equation above.
(a) Write the equilibrium-constant expression for the dissociation of HF(aq) in water.
(b) Calculate the molar concentration of H3O+ in a 0.40 M HF(aq) solution.
HF(aq) reacts with NaOH(aq) according to the reaction represented below.
HF(aq) + OH–(aq)  H2O(l) + F–(aq)
A volume of 15 mL of 0.40 M NaOH(aq) is added to 25 mL of 0.40 M HF(aq) solution. Assume that volumes are
additive.
(c) Calculate the number of moles of HF(aq) remaining in the solution.
(d) Calculate the molar concentration of F–(aq) in the solution.
(e) Calculate the pH of the solution.
2007 A form B
1.
A sample of solid U308 is placed in a rigid 1.500 L flask. Chlorine gas, Cl2(g), is added, and the flask is
heated to 862˚C. The equation for the reaction that takes place and the equilibrium-constant expression
for the reaction are given below.
U308(s) + 3 Cl2(g),  3 UO2Cl2(g) + O2(g) K P 
(PUO2 Cl2 )3 (PO2 )
(PCl2 )3
When the system is at equilibrium, the partial pressure of Cl2(g) is 1.007 atm and the partial pressure of
UO2Cl2(g) is 9.73410-4 atm
(a) Calculate the partial pressure of O2(g) at equilibrium at 862˚C.
(b) Calculate the value of the equilibrium constant, KP, for the system at 862˚C.
(c) Calculate the Gibbs free-energy change, ∆G˚, for the reaction at 862˚C.
(d) State whether the entropy change, ∆S˚ for the reaction at 862˚C is positive, negative, or zero. Justify your
answer.
(e) State whether the enthalpy change, ∆H˚, for the reaction at 862˚C is positive, negative, or zero. Justify
your answer.
(f)
After a certain period of time, 1.000 mol of O2(g) is added to the mixture in the flask. Does the mass of
U308(s) in the flask increase, decrease, or remain the same? Justify your answer.
2006 A
Answer the following questions that relate to solubility of salts of lead and barium.
(a) A saturated solution is prepared by adding excess PbI2(s) to distilled water to form 1.0 L of solution at
25˚C. The concentration of Pb2+(aq) in the saturated solution is found to be 1.3  10–3 M. The chemical
equation for the dissolution of PbI2(s) in water is shown below.
PbI2(s)  Pb2+(aq) + 2 I–(aq)
(i) Write the equilibrium-constant expression for the equation.
(ii) Calculate the molar concentration of I–(aq) in the solution.
(iii) Calculate the value of the equilibrium constant, Ksp.
(b) A saturated solution is prepared by adding PbI2(s) to distilled water to form 2.0 L of solution at 25˚C. What
are the molar concentrations of Pb2+(aq) and I–(aq) in the solution? Justify your answer.
(c) Solid NaI is added to a saturated solution of PbI2 at 25˚C. Assuming that the volume of the solution does
not change, does the molar concentration of Pb2+(aq) in the solution increase, decrease, or remain the
same? Justify your answer.
(d) The value of Ksp for the salt BaCrO4 is 1.2  10–10. When a 500. mL sample of 8.2  10–6 M Ba(NO3)2 is
added to 500. mL of 8.2  10–6 M Na2CrO4, no precipitate is observed.
(i)
(ii)
Assuming that volumes are additive, calculate the molar concentrations of Ba2+(aq) and CrO42–(aq) in
the 1.00 L of solution.
Use the molar concentrations of Ba2+(aq) ions and CrO42-(aq) ions as determined above to show why
a precipitate does not form. You must include a calculation as part of your answer.
2005 A
HC3H5O2(aq) ↔ C3H5O2–(aq) + H+(aq)
Ka
–5
Propanoic acid, HC3H5O2, ionizes in water according to the equation above.
(a) Write the equilibrium constant expression for the reaction.
(b) Calculate the pH of a 0.265 M solution of propanoic acid.
(c) A 0.496 g sample of sodium propanoate, NaC3H5O2, is added to a 50.0 mL sample of a 0.265 M solution
of propanoic acid. Assuming that no change in the volume of the solution occurs, calculate each of the
following.
(i)
The concentration of the propanoate ion, C3H5O2–(aq) in the solution
The concentration of the H+(aq) ion in the solution.
The methanoate ion, HCO2–(aq) reacts with water to form methanoic acid and hydroxide ion, as shown in the
(ii)
following equation.
HCO2–(aq) + H2O (l) ↔ H2CO2(aq) + OH–(aq)
(d) Given that [OH–
following.
–6 M in a 0.309 M solution of sodium methanoate, calculate each of the
(i)
The value of Kb for the methanoate ion, HCO2–(aq)
(ii)
The value of Ka for methanoic acid, HCO2H
(e) Which acid is stronger, propanoic acid or methanoic acid? Justify your answer.
2004 A Required
Answer the questions relating to the solubility’s of two silver compounds, Ag2CrO4 and Ag3PO4
Silver chromate dissociates in water according to the following equation:
Ag2CrO4(s)
+(aq) + CrO42–(aq)
–12 at 25˚C
Ksp
(a) Write the equilibrium constant expression for the dissolving of Ag2CrO4.
(b) Calculate the concentration in mol L–1, of Ag+ in a saturated solution of Ag2CrO4 at 25˚C.
(c) Calculate the maximum mass in grams of Ag2CrO4 that can dissolve in 100. mL of water at 25˚C.
(d) A 0.100 mol sample of solid AgNO3 is added to 1.00 L saturated solution of Ag2CrO4. Assuming no volume
change, does [CrO42–] increase, decrease, or remain the same? Justify your answer.
In a saturated solution of Ag3PO4 at 25˚C, the concentration of Ag+
expression for the dissolving of Ag3PO4 is shown below:
–5 M. The equilibrium constant
Ksp = [Ag+]3[PO43–]
(e) Write the balanced equation for the dissolving of Ag3PO4 in water.
(f)
Calculate the value of Ksp for Ag3PO4 in water.
(g) A 1.00 L sample of saturated Ag3PO4 solution is allowed to evaporate at 25˚C to a final volume of 500.
mL. What is [Ag+] in the solution? Justify your answer.
2003 A
C6H5NH2(aq) + H2O(l) ↔ C6H5NH3+(aq) + OH–(aq)
Aniline, a weak base, reacts with water according to the reaction represented above.
(a) Write the equilibrium constant expression, Kb, for the reaction represented above.
(b) A sample of aniline is dissolved in water to produce 25.0 mL of 0.10 M solution. The pH of the solution is
8.82. Calculate the equilibrium constant, Kb, for this reaction.
(c) The solution prepared in part (b) is titrated with 0.10 M HCl. Calculate the pH of the solution when 5.0 mL
of the acid has been titrated.
(d) Calculate the pH at the equivalence point of the titration in part (c).
(e) The pKa values for several indicators are given below. Which of the indicators listed is most suitable for
this titration? Justify your answer.
Indicator
pKa
Erythrosine
3
Litmus
7
Thymolphthalein 10
2002 A
HOBr(aq) ↔ H+(aq) + OBr–(aq)
Ka
–9
Hypobromous acid, HOBr, is a weak acid that dissociates in water, as represented by the equation above.
(a) Calculate the value of [H+] in an HOBr solution that has a pH of 4.95.
(b) Write the equilibrium constant expression for the ionization of HOBr in water, then calculate the
–5 M.
concentration of HOBr(aq) in an HOBr solution that has [H+
(c) A solution of Ba(OH)2 is titrated into a solution of HOBr.
(i)
(ii)
Calculate the volume of 0.115 M Ba(OH)2(aq) needed to reach the equivalence point when titrated
into a 65.0 mL sample of 0.146 M HOBr(aq).
Indicate whether the pH at the equivalence point is less than 7, equal to 7, or greater than 7. Explain.
(d) Calculate the number of moles of NaOBr(s) that would have to be added to 125 mL of 0.160 M HOBr to
–9 M. Assume that volume change is negligible.
produce a buffer solution with [H+] = 5.00
(e) HOBr is a weaker acid than HBrO3. Account for this fact in terms of molecular structure.
2001 A
Answer the following questions relating to the solubility of the chlorides of silver and lead.
(a) At 10C, 8.9  10-5 g of AgCl(s) will dissolve in 100. mL of water.
(i) Write the equation for the dissociation of AgCl(s) in water.
(ii) Calculate the solubility, in mol L–1, of AgCl(s) in water at 10C.
(iii) Calculate the value of the solubility-product constant, Ksp for AgCl(s) at 10C.
(b) At 25C, the value of Ksp for PbCl2(s) is 1.6  10-5 and the value of Ksp for AgCl(s) is 1.8  10-10.
(i)
If 60.0 mL of 0.0400 M NaCl(aq) is added to 60.0 mL of 0.0300 M Pb(NO3)2(aq), will a precipitate
form? Assume that volumes are additive. Show calculations to support your answer.
(ii) Calculate the equilibrium value of [Pb2+(aq)] in 1.00 L of saturated PbCl2 solution to which 0.250 mole
of NaCl(s) has been added. Assume that no volume change occurs.
(iii) If 0.100 M NaCl(aq) is added slowly to a beaker containing both 0.120 M AgNO3(aq) and 0.150 M
Pb(NO3)2(aq) at 25C, which will precipitate first, AgCl(s) or PbCl2(s)? Show calculations to support
your answer.
2000 A Required
2 H2S(g)  2 H2(g) + S2(g)
When heated, hydrogen sulfide gas decomposes according to the equation above. A 3.40 g sample of H2S(g) is
–2
mol of S2(g) is present at equilibrium.
(a) Write the expression for the equilibrium constant, Kc, for the decomposition reaction represented above.
(b) Calculate the equilibrium concentration, in molL-1, of the following gases in the container at 483 K.
(i)
(ii)
H2(g)
H2S(g)
(c) Calculate the value of the equilibrium constant, Kc, for the decomposition reaction at 483 K.
(d) Calculate the partial pressure of S2(g) in the container at equilibrium at 483 K.
1
(e) For the reaction H2(g) + 2 S2(g)  H2S(g) at 483 K, calculate the value of the equilibrium constant, Kc.