328 Report Sheet Hydrolysis of salts and pH of Buffer solutions QUESTIONS i. Usi

Question

328 Report Sheet Hydrolysis of salts and pH of Buffer solutions QUESTIONS i. Using the Ka's for Hc2H,o, and Hco, (from Appendix F), calculate the KM's for the ChHyo2 and CO ions. Compare these values with those calculated from your measured pH's. 2. Using K for NH, (from Appendix G), calculate Ka for the NH4 Compare this value with that calcu- ion. lated from your measured pH's 3. How should the pH of a 0.1 M solution of NaC2H,02 compare with that of a 0.1 M solution of Kc2Hyor? Explain briefly. 4, what is the greatest source of error in this experiment? How could you minimize this source of error? rnmffer Solutions

Explanation / Answer

The weak acid HA and its conjugate base is A-.

The Ka and Kb of the acid andits conjugate base are related as

Ka of acid x Kb of conjugae base = Kw = 1.0x10-14

1) Ka of HC2H3O2 = 1.8x10-5

Thus Kb of C2H3O2- = Kw/Ka =

= 1.0x10-14 /1.8x10-5

= 5.55x10-10

similarly Kb of CO3-2 = Kw/Ka of HCO3-

= 1.0x10-14 /5.6x10-11

   = 1.785x10-4

Q2)similarly Kb of weak base and Ka of its conjugate acid are related as

Kb (weak base) x Ka of its conjugate acid = Kw

Thus Kaof NH4+ = Kw/Kb

=1.0x10-14 /1.8x10-5

= 5.55x10-10

3) Both NaC2H3O2 and KC2H3O2 are salts of weak acid, acetic acid with strong bases.

As the anion only underges hydrolysis, the cation (Na+ or K+) of strong base does not affect the pH of such salt solution.

The pH of such salt is given by

pH = 1/2 [pkw +pKa + logC]

= 1/2[14 + 4.76 + log 0.1]

= 8.88

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