For each of the following, is the solution acidic, basic, neutral, or cannot be determined?

Question

(For each, write the equation for the dominant equilibrium which determined the pH, and justify your pH prediction)

(a) 100 mL of 0.10 M C5H5N; Kb = 1.5x10^-9

(b) 100 mL of 0.10M Ca(NO3)2

(c) 100mL of 0.10 M NaH2PO4; Ka1 = 7.1 x 10^-3; Ka2 = 6.3 x 10^-8; Ka3 = 4.2 x 10^-13

(d) 100 mL of 0.10 M K2PO4; Ka = See above.

(e) 100mL of 0.100 M Na3PO4; Ka = see above.

Answer 1

1. pyradine makes a basic solution.
2. calcium nitrate should be neutral.
3. Phosphoric acid has 3 Ka, Ka1 indicating the point between the acid and the dihydrogen phosphat, Ka2 the point between the dihydrogen phosphate and the monohydrogen phosphate, and finally, Ka3, the point between the monohydrogen phosphate and the phosphate ions. The reaction to follow is
[HxPO4]-(3-x) + H20 -> [Hx+1 PO4]-(2-x)] +OH- where x can be 0, 1, or 2
The equilbrium constant for the reaction can be derived from the appropiate Ka as
[HxPO4]/{ [Hx+1PO4][OH-] } = Ka(x+1)/Kw, where Kw = 1x10^-14. You can do the math for (c) and find that this is basic. Thus, the others will also be basic.