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# Aqueous Equilibria and Chemical Thermodynamics

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1. Calculate the pH of
a) A solution that is 0.160 M in sodium formate(NaCHO2) and 0.260 M in formic acid(HCHO2).
b) A solution that is 0.210 M in pyridine(C5H5N) and 0.350 M in pyridinium chloride(C5H5NHCl).
c) A solution that is made by combining 125mL of 0.050 M hydrofluoric acid with 50.0mL of 0.10 M sodium fluoride.

2. a) Calculate the percent ionization of 0.085 M lactic acid(Ka= 1.4 x 10^-4)
b) Calculate the percent ionization of 0.085 M lactic acid in a solution containing 0.050M sodium lactate.

3. A buffer solution contains 0.12mol of propionic acid(HC3H5O2) and 0.10mol of sodium propionate(NaC3H5O2) in 1.50 L.
a) What is the pH of this buffer?
b) What is the pH of the buffer after the addition of 0.01mol of NaOH?
c) What is the pH of the buffer after the addition of 0.01mol of HI?

4. Consider the titration of 30.0mL of 0.030 M NH3 with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added:
a) 0mL
b) 10.0mL
c) 20.0mL
d) 35.0mL
e) 36.0mL
f) 37.0mL

5. A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2+ and 0.010 M in Sr2+.
a) What concentration of [SO4 2-] is necessary to begin precipitation? (Neglect volume changes. BaSO4: Ksp= (1.1 x 10^-10); SrSO4: Ksp= (3.2 x 10^-7)
b) Which cation precipitates first?
c) What is the concentration of [SO4 2-] when the second cation begins to precipitate?

6. Using the value of Ksp for Ag2S, Ka1, and Ka2 for H2S,and Kf= (1.1 x 10^5) for AgCl2-, calculate the equilibrium constant for the following reaction:
[Ag2S(s)] + [4 Cl-(aq)] + [2 H+(aq)] -----> [2 AgCl2-(aq)] + [H2S(aq)]
<----

7. Potassium hydrogen phthalate, often abbreviated KHP, can be obtained in high purity and is used to determine the concentrations of solutions of strong bases. Strong bases react with the hydrogen phthalate ion as follows: [HP-(aq)] + [OH-(aq)]-----> [H2O(l) + [P2-(aq)]

The molar mass of KHP is 204.2 g/mol and Ka for the Hp- ion is 3.1 x 10^-6.
a) If a titration experiment begins with 0.4885g of KHP and has a final volume of about 100mL, which indicator would be most appropriate?
b) If the titration required 38.55mL of NaOH solution to reach the end point, what is the concentration of the NaOH solution?

8. The element cesium(Cs) freezes at 28.4 degrees Celsius, and its molar enthalpy of fusion is ∆ Hfus= 2.09 kJ/mol
a) When molten cesium solidifies to Cs(s) at its normal melting point, is entropy change(∆ S) positive or negative?
b) Calculate the value of (∆ S) when 15.0g of Cs(l) solidifies at 28.4 degrees Celsius.

9. Predict the sign of ,(∆ S)system, for each of the following processes:
a) Molten Fe solidifies
b) LiCl(s) is formed from Li(s) and Cl2(g).
c) Zinc metal dissolves in hydrochloric acid, forming ZnCl2(aq), and H2(g)
d)Silver bromide precipitates upon mixing AgNO3(aq) and KBr(aq)

10. Calculate ∆ Sdegree values for the following reactions: (in each case explain the sign of ∆ Sdegree)
a) [N2H4(g)] + [H2(g)]----->[2 NH3(g)]
b) [2 Al(s)] + [3 Cl2(g)]--->[2 AlCl3(s)]
c) [Mg(OH)2(s)] + [2 HCl(g)]--->[MgCl2(s)] + [2 H2O(l)]
d) [2 CH4(g)]---->[C2H6(g)] + [H2(g)]

11. From the values given for ∆ Hdegree and ∆ Sdegree, calculate ∆ Gdegree for each of the following reactions at 298K. If the reaction is not spontaneous under standard conditions at 298K, at what temperature (if any), would the reaction become spontaneous?
a) [2 PbS(s)] + [3 O2(g)]-----> [2 PbO(s)] + [2 SO2(g)]
∆ Hdegree= -844kJ ∆ Sdegree= -165 J/K

b) [2 POCl3(g)]---->[2 PCl3(g)] + [O2(g)]
∆ Hdegree= 572 kJ ∆ Sdegree= 179 J/K

12. The Kb for methylamine(CH3NH2) at 25 degrees Celsius is 4.4 x 10^-4.
a) Write the chemical equation for the equilibrium that corresponds to Kb.
b) By using the value of Kb, calculate ∆ Gdegree for the equilibrium in part a.
c) What is the value of ∆ G at equilibrium?
d) What is the value of ∆ G when [H+]= 1.5 x 10^-8 M, [CH3NH3+]= [H+]= 1.5 x 10^-8 M, [CH3NH3+]= 5.5 x 10^-4 M, and [CH3NH2]= 0.120 M?

13. Consider the following three reactions:
i)[2 RbCl(s)] + [3 O2(g)]----> [2 RbClO3(s)]
ii) [C2H2(g)] + [4 Cl2(g)]--->[2 CCl4(l)] + [H2(g)]
iii) [TiCl4(l)] + [2 H2O(l)]--->{TiO2(s)] + [4 HCl(aq)]
a) For each of the reactions calculate ∆ H, ∆ G, and ∆ S at 25degrees Celsius.
b) Which of these reactions are spontaneous under standard conditions at 25 degrees Celcius?
c) For each of the reactions, predict the manner in which the change in free energy varies with an increase in temperature.

https://brainmass.com/chemistry/energetics-and-thermodynamics/aqueous-equilibria-chemical-thermodynamics-282748

#### Solution Summary

This solution assists with questions regarding aqueous equilibria and chemical thermodynamics.

\$2.19

## Acid-Base Reactions, Buffer and Thermodynamics

17.16) Use information from Appendix D to calculate the pH of (a) a solution that is 0.250 M in sodium formate (HCOONa) and 0.100 M in formic acid (HCOOH); (b) a solution that is 0.510 M in pyridine (C5H5N) and 0.450 M in pyridinium chloride (C5H5NHCl); (c) a solution that is made by combining 55 mL of 0.050 M hydrofluoric acid with 125 mL of 0.10 M sodium fluoride.

For Appendix D see:
https://online.vitalsource.com/#/books/9781269447980/content/id/app04
pg 1062

17.18) (a) Calculate the percent ionization of 0.125 M lactic acid (Ka = 1.4 × 10−4). (b) Calculate the percent ionization of 0.125 M lactic acid in a solution containing 0.0075 M sodium lactate.

17.28) A buffer contains 0.15 mol of propionic acid (C2H5COOH) and 0.10 mol of sodium propionate (C2H5COONa) in 1.20 L. (a) What is the pH of this buffer? (b) What is the pH of the buffer after the addition of 0.01 mol of NaOH? (c) What is the pH of the buffer after the addition of 0.01 mol of HI?

17.46) Consider the titration of 30.0 mL of 0.050 M NH3 with 0.025 M HCl. Calculate the pH after the following volumes of titrant have been added: (a) 0 mL, (b) 20.0 mL, (c) 59.0 mL, (d) 60.0 mL, (e) 61.0 mL, (f) 65.0 mL.

17.66) Using the value of Ksp for Ag2S, Ka1 and Ka2 for H2S, and Kf = 1.1 × 105 for AgCl2−, calculate the equilibrium constant for the following reaction:
Ag2S(s) + 4 Cl−(aq) + 2 H+(aq) ⇌ 2 AgCl2−(aq) + H2S(aq)

17.72) A solution of Na2SO4 is added dropwise to a solution that is 0.010 M in Ba2+ and 0.010 M in Sr2+. (a) What concentration of SO42− is necessary to begin precipitation? (Neglect volume changes. BaSO4: Ksp = 1.1 × 10−10; SrSO4: Ksp = 3.2 × 10−7.) (b) Which cation precipitates first? (c) What is the concentration of SO42− when the second cation begins to precipitate?

17.89) A sample of 0.1687 g of an unknown monoprotic acid was dissolved in 25.0 mL of water and titrated with 0.1150 M NaOH. The acid required 15.5 mL of base to reach the equivalence point. (a) What is the molecular weight of the acid? (b) After 7.25 mL of base had been added in the titration, the pH was found to be 2.85. What is the Ka for the unknown acid?

________________________________________________________________________

19.26) The element gallium (Ga) freezes at 29.8°C, and its molar enthalpy of fusion is ΔHfus = 5.59 kJ/mol. (a) When molten gallium solidifies to Ga(s) at its normal melting point, is ΔS positive or negative? (b) Calculate the value of ΔS when 60.0 g of Ga(l) solidifies at 29.8°C.

19.44) Predict the sign of ΔSsys for each of the following processes: (a) Molten gold solidifies. (b) Gaseous Cl2 dissociates in the stratosphere to form gaseous Cl atoms. (c) Gaseous CO reacts with gaseous H2 to form liquid methanol, CH3OH. (d) Calcium phosphate precipitates upon mixing Ca(NO3)2(aq) and (NH4)3PO4(aq).

19.54) Calculate ΔS° values for the following reactions by using tabulated S° values from Appendix C. In each case explain the sign of ΔS°.
(a) HNO3(g) + NH3(g) → NH4NO3(s)
(b) 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g)
(c) CaCO3(s, calcite) + 2HCl(g) → CaCl2(s) + CO2(g) + H2O(l)
(d) 3 C2H6(g) → C6H6(l) + 6 H2(g)

19.66) From the values given for ΔH° and ΔS°, calculate ΔG° for each of the following reactions at 298 K. If the reaction is not spontaneous under standard conditions at 298 K, at what temperature (if any) would the reaction become spontaneous?

19.86) The Kb for methylamine (CH3NH2) at 25°C is given in Appendix D. (a) Write the chemical equation for the equilibrium that corresponds to Kb. (b) By using the value of Kb, calculate ΔG° for the equilibrium in part (a). (c) What is the value of ΔG at equilibrium? (d) What is the value of ΔG when [H+] = 6.7 × 10-9 M, [CH3NH3+] = 2.4 × 10-3 M, and [CH3NH2] = 0.098 M?

19.97) Consider the following three reactions:
(i) Ti(s) + 2 Cl2(g) → TiCl4(g)
(ii) C2H6(g) + 7 Cl2(g) → 2 CCl4(g) + 6 HCl(g)
(iii) BaO(s) + CO2(g) → BaCO3(s)
(a) For each of the reactions, use data in Appendix C to calculate ΔH°, ΔG°, and ΔS° at 25°C. (b) Which of these reactions are spontaneous under standard conditions at 25°C? (c) For each of the reactions, predict the manner in which the change in free energy varies with an increase in temperature.

For appendix C See:
https://online.vitalsource.com/#/books/9781269447980/content/id/app03
pg 1059

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