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Buffer Solution and Henderson-Hasselbalch (H-H)

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13. What is the pH of a solution prepared by mixing 50.00mL of 0.10 M NH3 with 25.00mL of 0.10 M NH4Cl? Assume that the volume of the solutions are additive and that Kb= 1.8 x 10 -5 for NH3.

14. What is the pH of a solution made by mixing 30.00mL of 0.10M acetic acid with 30.00mL of 0.10 M KOH? Assume that the volumes of the solutions are additive. Ka=1.8 x 10-5 for CH3CO2H.

15. What is the pH of a solution prepared by mixing 25.00mL of 0.10 M Ch3CO2H with 25.00mL of 0.050 M CH3CO2Na? Assume that the volume of the solutions are additive and that Ka=1.8 x 10-5 for CH3CO2H.

16. For the reaction 3C2H2(g) yields C6H6(l) at 25 degrees Celcius, the standard enthalpy change is -631 kJ and the standard entropy change is -430 J/K. Calculate the standard free energy change at 25 degrees Celcius.

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Solution Summary

Chemistry problems involving buffers, thermodynamic equation, and the Henderson-Hasselbalch (H-H) equation.

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13. What is the pH of a solution prepared by mixing 50.00mL of 0.10 M NH3 with 25.00mL of 0.10 M NH4Cl? Assume that the volume of the solutions are additive and that Kb= 1.8 x 10 -5 for NH3.

First we need to know the moles of each species involved:
NH3: 50.00* 10-3* 0.10 M =5.0* 10-3
NH4Cl: 25.00* 10-3* 0.10 M = 2.5 * 10-3

Then we calculate the molarities:
5.0* 10-3 moles/75 * 10-3 L =0.67
2.5 * 10-3 moles/75 * 10-3 L =0.33

Ka and Kb are related by:
pKa = 14 - pKb
If Kb is 1.8 *10-5
then pKb is - log (1.8 *10-5)=4.74
Therefore
pKa =14 -4.74 =9.26

Using the Henderson-Hasselbalch (H-H) equation,
pH = pKa + log (0.67)/(0.33) = 9.26+ log 2 =9.26 + 0.30 = 9.56

14. What is the pH of a solution made by mixing 30.00mL of 0.10M acetic acid with 30.00mL of 0.10 M KOH? Assume ...

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