Finding the best Indicator.....?

Question

Can someone please explain the method behind these answers?

How do you find the endpoint, etc.?

Thanks!

Which of the following indicators would be most suitabe for the titration of .10 M (CH3)3N

with .1M HClO4? For trimethyamine, pKb = 4.19

The answers are indicators with pKins of

pKin = 11.12

pKin = 9.4

pKin = 7.1

pKin = 1.7

pKin = 4.7

Here's another one:

Which of the following indicators would be most sitable for the titration of .1M lactic acid with .1M KOH? For lactic

acid, pKa = 3.08

The answers are indicators with pKins of

pKin = 1.7

pKin = 7.9

pKin = 3.9

pKin = 3.4

pKin = 11.2

Answer 1

I guess the one with the highest! better look it up in the google.

Answer 2

Indicators are weak acids (the have a hydrogen the can lose).

Therefore, HA is an undissociated acid, HInd is and undissociated indicator molecule. H+ is a hydrogen, Ind-/A- are the conjugal bases.

So, when you add an indicator to an acid (a solution with a preternaturally high [H+]), by Le Chatalier's Principle (LCP) of equilibria that the HInd molecules will not dissociate according to HInd <==> H+ + Ind- because the [H+] (concentration of H+) is high and forces the equilibrium to favour a right-to-left motion. However, as [H+] decreases, more and more HInd dissociates (by LCP again) until when alkaline conditions are reached (a prevelance of OH-) the dissociation is actively favoured and the majority of the indicator becomes the Ind- form, which is the colour change ( which occurs due to all kinds of physical chem magic, but would take too many years to explain in type ).

Assuming you understand this principle, the ideas of pKa and pKind are easy. As with a pKa, which is the pH at which [HA]= ~[A-] (the equivalence point), pKind is the pH at which [HInd]= ~[Ind-].  So, in short you want to choose the indicator with the closest equivalence point to that of your reaction - so 4.7 in the first instance, and 3.4 in the second. If you want any more clarification, leave an edit or message me.