279 Date, Lab Section Results and Discussion - Experiment 23- Acids and Bases: p

Question

279 Date, Lab Section Results and Discussion - Experiment 23- Acids and Bases: pH Measurements o an extemsion to this experiment, sce Activity 35, Part C A pH of acids, bases and salts pH (predicted) pH (paper) pH (electronic) # Solution 1. 0.1 MHC? 2. 0.01 MHC? 3. 0.001 M HCI 4. 0.1 M HC,H,O 5. 0.1 MNH,CL 6. deionized water 7 tap water 8. 0.1 M NaCI 9. 0.1 M NaC,H,O, 1.0 291 s.04 5.15 1.80 3 10. 0.1 M NaHCO, 11. O. 1 M Na2CO3 12. 0.1 MNH, 13. 0.1 M NaOH 14. Compare and explain the pH differences between solutions I and 4. 7.59 3.7 11.32 0.72 l.05 15. Do the pH values for solutions 1, 2 and 3 indicate that pH approaches log.olH as the concentration of H decreases? Explain your answer. Use the concentrations and pH values for solution 4 to approximate K, for acetic acid. 16. O2015 Cengage Learming All Rights Reserved. Many not be scanned, copied or duplicated, ce posted to a publicly accessible websine, in whole or in part

Explanation / Answer

14.) HCl is a strong acid so it will ionise readily as compared to HC2H3O2 which is a weak acid and uses a bit different approach.

For strong acid we use direct formula pH = - log[H+]. Hence it is used for HCl

But for weak acids we first find 'Kb' { Kb = Kw / Ka }

then [H+] concentration for weak acids is (Kb x Molarity)1/2

Thats why there is a difference between the pH of HCl and HC2H3O2.

15.) Yes, the pH values for solutions 1, 2, and 3 indicates that pH approaches - log [H+] as the concentration of H+ decreases. Thats because the acid used is HCl which is a strong acid and gets ionised in the solution readily. So direct approach can be used to find the pH of such acids.

HCl ---> H+ + Cl-

16.) We know that;

given pH of HC2H3O2 is 2.91

so, pH = - log [H+]

2.91 = - log [H+]

[H+] = 1.23 x 10-3

Now;

  [H+] concentration for weak acids is (Kb x Molarity)1/2

  1.23 x 10-3 = (Kb x 0.1)1/2

Kb = 1.51 x 10-5

we know that

Kb = Kw / Ka

1.51 x 10-5 = 10-14 / Ka

Ka = 6.6 x 10-10

Hence this is the value of Ka for acetic acid.

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