a) Use what you know about titration curves to calculate the pKa for this first

Question

a) Use what you know about titration curves to calculate the pKa for this first proton.

b) What is Ka for this first proton? (also called Ka1)

1. Diprotic Acids and Bases

Polyprotic acids have more than one acidic proton. These can be lost in a stepwise fashion where each subsequent proton is less acidic than the one lost before it. We can see this in the titration curve of phosphoric acid shown below.

We can see the first equivalence point happens after 10.0 mL of the strong base has been added.

a) Use what you know about titration curves to calculate the pKa for this first proton.

b) What is Ka for this first proton? (also called Ka1)

The second equivalence point is at 20.0 mL.
c) What is the Ka (Ka2) for this second proton? H,POtitrated with 0.10M NaOH 5 10 15 20 25 30 35 E 35 10 15 20 25 30 mL of 0.10 M NaOH added

Explanation / Answer

A:- (a) At half equivalence point the pH = pKa1

pH = 2.3

pKa1 = 2.3

(b) Ka1 = 10^-2.3

     Ka1 = 5.01 * 10^-3

(c) At second half equivalence point, pH = pKa2

pH = 7.2 = pKa2

pKa2= 7.2

Ka2 = 10^-7.2

Ka2 = 6.31 * 10^-8

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