Buffers Question! TBR CHALLENGE

[SonhosDaVida]

Two questions:

In a buffer, if plotted on titration graph, it'd be a straight line with pH = pKa?

Also, for this problem(#9), why do u need to have an indicator pKa less than 7? Since this is a weak base, NH3 diluted by strong acid HCl, then the pH at equivalence is less than 7.
So for indicator active range, it should be plus or minus the pH at the equivalence point. Thus, since a pH lower than 7 would be 6, the indicator can only have a pKa of 5-7? Hence H-IndK (pka 7.43) is too great and not the correct answer? Can someone verify my thinking on this? Thanks

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[SonhosDaVida]

I was wondering if anyone can shed some light on this this problem? Thank you

 

[Lawpy]

In a buffer, if plotted on titration graph, it'd be a straight line with pH = pKa?

Yep but only for a certain amount of acid or base added. Buffer solutions can only resist small changes in pH.

Also, for this problem(#9), why do u need to have an indicator pKa less than 7? Since this is a weak base, NH3 diluted by strong acid HCl, then the pH at equivalence is less than 7.
So for indicator active range, it should be plus or minus the pH at the equivalence point. Thus, since a pH lower than 7 would be 6, the indicator can only have a pKa of 5-7? Hence H-IndK (pka 7.43) is too great and not the correct answer? Can someone verify my thinking on this? Thanks

View attachment 239653 View attachment 239654

You have the right idea. NH3 is a weak base and HCl is a strong acid, so combining them would result in a solution that's acidic (and thus having an equivalence point pH < 7). So you would choose an indicator whose pKa is less than 7 in order to detect the equivalence point of the solution. Table 1 shows that only H-IndK has a pKa < 7 so it's best suited for the NH3-HCl titration.

Since we're titrating a weak base with a strong acid, we're starting off with a basic NH3 solution and gradually adding more HCl. This means we're adding more protons to the basic solution and thus NH3 gradually gains protons to become positively charged/protonated NH4+. This means this titration results in changing NH3 (deprotonated form --> without protons) to NH4+ (protonated form --> with protons), so the color of the indicator changes from chartreuse (representing deprotonated state) to teal (representing protonated state).

acid-base indicators