Using the Ka\'s for HC_2H_3O_2 and HCO_3^- (from Appendix E in the back of this

Question

Using the Ka's for HC_2H_3O_2 and HCO_3^- (from Appendix E in the back of this manual), calculate the Kb's for the C_2H_3O_2^- and CO_3^2- ions. Compare these values with those calculated from your measured pH's. 2. Using Kb, for NH_3 (from Appendix E), calculate Ka for the NH_4^+ ion. Compare this value with that calculated from your measured pH's. 3. Compare the pH of a 0.1 M solutions of Ca(C_2H_3O_2)_2, Na(C_2H_3O_2) and K(C_2H_3O_2) ? Explain briefly. 4. The greatest source of error in this experiment is the uncertainty of the pH measurements based on the use of indicator color. How could you make more accurate pH measurements?

Explanation / Answer

I´m obligated to respond at least one question if they are not directly related, I will answer item 1 and item 2 as a bonus, but please post another question for the last two items and someone will gladly answer them.

As you do not provide us with the Appendix E I will use the values of Ka and Kb from a universitary book:

1. For this exercise we need to know that Kw = Ka x Kb , being Kw = 10-14

HC2H3O2 (acetic acid) Ka= 1.76 x 10-5

Kw = Ka x Kb ---------------------- Kb= Kw/Ka -------------------Kb= 10-14/1.76 x 10-5 = 5.68x10-10

HCO3- (Hydrogen carbonate ion) Ka = 4.8x10-11

Kw = Ka x Kb ---------------------- Kb= Kw/Ka -------------------Kb= 10-14/4.8 x 10-11 = 2.08x10-4

2. NH3 (ammonium) Kb=1.8x10-5

Kw = Ka x Kb ---------------------- Ka= Kw/Kb -------------------Ka= 10-14/1.8 x 10-5 = 5.55x10-10

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