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Thermodynamics and pH

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1) A quantity of 7.480g of an organic compound is dissolved in water to make 300.0 mL of solution. The solution has an osmotic pressure of 1.43 atm at 27 degrees Celsius. The analysis of this compound shows that it contains 41.8 percent C, 4.7 percent H, 37.3 percent O and 16.3 percent N. Calculate the molecular formula of the compound.

2) The Ka for benzoic acid is 6.5 x 10^-5. Calculate the pH of a 0.10 M benzoic acid solution.

3) If 20.0 mL of 0.10 M Ba(NO3)2 are added to 50.0 mL of 0.10 M Na2CO3, will BaCO3 precipitate?

4) Compare the molar solubility of Mg(OH)2 in water and in a solution buffered at a pH of 9.0.

5) Calculate the pH of the 0.20M NH3/0.20M NH4Cl buffer. What is the pH of the buffer after the addition of 10.0 mL of 0.10M HCl to 65.0 mL of the buffer?

6) Given the following absolute entropies, determine So for the reaction

SO3(g) + H2O(l) H2SO4(l)
So (J/K mol)
SO3 256.2
H2O 69.9
H2SO4 156.9

7) Given the following free energies of formation, calculate Go for the reaction

3NO2(g) + H2O(l) 2HNO3(l) + NO(g)
G (kJ/mol)
H2O(l) -237.2
HNO3(l) -79.9
NO(g) 86.7
NO2(g) 51.8

8) Nitrosyl chloride (NOCl) decomposes at elevated temperatures according to the equation

2NOCL(g) 2NO(g) + Cl2(g)

Use the following information to calculate KP for this reaction at 227oC:

Ho = 81.2 kJ
So = 128 J/K

9) Given the following free energies of formation, calculate KP for the reaction below at 298 K.

SO2(g) + NO2(g) SO3(g) + NO(g)
G (kJ/mol)
SO2(g) -300.4 kJ/mol
SO3(g) -370.4 kJ/mol
NO(g) 86.7 kJ/mol
NO2(g) 51.8 kJ/mol

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Solution Summary

This solution explains:

1) How to calculate the molecular formula of a compound based on percent composition.
2) How to calculate the pH of a weak acid.
3) How to calculate molar solubility.
4) How to calculate the pH of a buffer solution.
5) How to use Hess's law to calculate enthalpy and free energy of reactions.
6) How to calculate the equilibrium constant of a reaction using Gibbs free energy.

See Also This Related BrainMass Solution

Aqueous Equilibria and Chemical Thermodynamics

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.

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