[kuromi]

C6H8O7 (aq)  +  OH- (aq)          C6H7O7- (aq)  +  H2O(l)

OR C6H8O7 (aq)  +  3OH- (aq)          C6H5O7 ^3- (aq)  +  3H2O(l)

Which one is correct ??   

[Borek]

Both, plus

C₆H₈O₇ +  2OH^-  C₆H₆O₇^2- + 2H₂O

Depends on what you are asking (or are asked) about.

[kuromi]

Both, plus

C₆H₈O₇ +  2OH^-  C₆H₆O₇^2- + 2H₂O

Depends on what you are asking (or are asked) about.

 to determine the Ka and pKa of 0.1M citric acid !! 
at 296 K , 20cm^3  of 0.1M citric acid titrate with 0.1M sodium hyoxide until end pt.
then , add a further 20cm^3  of  the same acid 
the pH of the mixture is 4.45  and used 62.7 cm^3 of sodium hyoxide  ~~~

[Borek]

to determine the Ka and pKa of 0.1M citric acid !!

pK_a1, pK_a2 or pK_a3?

at 296 K , 20cm^3  of 0.1M citric acid titrate with 0.1M sodium hyoxide until end pt.

Just out of curiosity - how do you determine end point?

then , add a further 20cm^3  of  the same acid 
the pH of the mixture is 4.45  and used 62.7 cm^3 of sodium hyoxide  ~~~

What will you have in the solution at this point?

Think about stoichiometry of the neutralization reaction. You already known citric acid is triprotic.

[kuromi]

this is 

to determine the Ka and pKa of 0.1M citric acid !!

pKa1, pKa2 or pKa3? --- > i dont know , lab sheet is just to ask the pKa !!

i used phenolphthalein --> end pt : colourless to pink

the pH of the mixture is 4.45  and used 62.7 cm^3 of sodium hyoxide for titration, 

which are my experimental results !!   

[Arkcon]

So, when the solution turns pink, what does that mean?  Is there any acid left?  What would it mean, to a citric acid molecule, if it had no acid left?  What is acid, anyway?

[Borek]

lab sheet is just to ask the pKa !!

No such thing as just pKa of the citric acid. It has to be one of the three.