Quant. Analysis In order for a titration to give a usably sharp end point, the t

Question

Quant. Analysis

In order for a titration to give a usably sharp end point, the titration reaction must be fast and must go to completion. Acid-base titrations should always use a strong titrant for this reason. (The reaction between a weak acid and a weak base will generally not have a large enough equilibrium constant to be considered “complete.”) Consider the titration of 10.0 mL of 0.100 M pyridinium chloride (the chloride salt of “HB+”—the conjugate acid of pyridine, “B”) with 0.100 M NaOH.

a. (1 pt) Write the net ionic equation for the titration reaction. How is the equilibrium constant for this reaction (call it Ktitr) related to Ka for HB+ or Kb for B? Why can you expect Ktitr to be “large”? Use data from Appendix G of Harris (http://www.csun.edu/~hcchm003/321/Ka.pdf) to calculate Ktitr.

Explanation / Answer

a)

Net ionic --> Typically only water...

Complete ionic -->

HB+(aq) + NaOH(Aq) --> Na+(aq) + B(aq) + H2O(l)

then

H+(aq) + OH(aq) --> H2O(l)

b)

Relate Ktitr with Ka for HB+ of Kb...

For

This weak base:

Ktitr = [B][H2O]/[HB+]

note that this is:

Kb = [HB+][OH-]/[B]

and

Kw = [H+][OH-] as well

expect this to be pretty low, since KW and Ka from Kb is pretty low

c)

Note that, after neutralization

"weak" base is left

i.e.

B + H2O <-> HB+ + OH-

then, this will be basic after neutralization

so:

pKb = 5.21

so

Kb = 10^-pKb = 10^-5.21

Kb = 6.165*10^-6

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