Na2CO3 titration with HCl?

Question

In a Na2CO3 titration with HCl I am supposed to do the following:

Calculate Ka2 (pH=pKa2 at 1/2 V equivalence point 1) and Ka1 (pH=pKa1 at 1/2 (V ep2 - V ep1)) for carbonic acid H2CO3.

I'm confused because of the 1/2 V eqivalence point volumes... I'm not sure how I would know what those are. From my titration curve I know that equivalence point one is at 9.90 mL HCl and 8.11 pH and that equivalence point two is 19.4 mL and 4.32 pH, but I don't know where to go from there.

Answer 1

Basic Principle  of  Acid - Base Titration Carce  

" pH  at  each  equivalent  point  is  equal  to  pKa  ( of  Acid ) "  

In  your case , The acid is Carbonic acid  ( H2CO3 )  

Starting from Carbonate   ;  CO3 (2-)  

At  First  equivalent  point  CO3 ( 2- )  - - >  HCO3 ( 1- )  and  pH  ( 8.11 ) equal  to  pKa2  ( of  Carbonic Acid )  

At  Second  equivalent  point  HCO3 ( 1- )  - - >  H2CO3  and  pH  ( 4.32 )  is  equal  to  pKa1  ( of  Carbonic Acid )  

I  do  agree  that  " the 1/2 V eqivalence point volumes "  make you confuse .  My suggestion is [[ Just forget them ! ]] .    

Answer 2

It's time for you to go to sleep and forget about this problem. Sweet dream!