Acid-Basestrength Pick the strongest base from the following list. a) PO 4 3- (H

Question

Acid-Basestrength
Pick the strongest base from the following list.

a) PO43- (H3PO4:Ka1 = 7.5 X 10-3; Ka2 = 6.2 X10-8; Ka3 = 4.2 X10-13)

b)NH3 (Kb = 1.8 X10-5)

c)N3- (Ka for HN3 = 1.9X 10-5)

d)NO3-

e)F- (Ka for HF = 6.8 X10-4)

I picked b, but it was wrong, I figured that the higherthe kb, the stronger the base, and the other questions had ka, so Idivided 1*10^4 / ka to get kb, which was still smaller than the kbfor b), so I choose b,

Is it a? and if so, how do I figure it out!? How do I approachthese kind of problems?

Explanation / Answer

The higher the Kb value, the stronger the base. You were right todivide 1.0e-14/ka = kb. NH3 is a strong base but PO43- is astronger base (higher kb). Look at the last Ka value.

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