[leena]

(1) I understand how  pH varies with the volume of HCl,i.e when we tirate HCl (in burette) and Na₂CO₃ (in the titration flask),using both methyl orange and phenolphthalein as indicators.
But can someone please explain how the pH would vary in a graph when Na₂CO₃(in burette) is titrated with HCl (in flask)

(2) Another basic question,
Before carrying out titrations,it's a must that we wash the burette and the pipette, first with distilled water  and if it's not dry ,with the solution that it's to be filled with,but,
it's not so with the titration flask.We start the titration even when there's some water left in the titration flask.
Why the difference?

[leena]

I may have figured this out,

In (2),it doesn't really matter if there's water left in the titration flask cause we've transferred 25 cm³ of the solution into the flask using the pipette,and the number of moles present doesn't change,so when we do the calculations, we just ignore the amount of water

For (1), Na2CO3 and HCl react in the volume ratio 1:2 ,so when a volume(say 25cm³) of HCl is in the flask only 12.5cm³ of Na2CO3 would be needed to neutralise so unlike in the 1st situation ,here(I think) there will only be one equivalence point.
I'm guessing ,the pH would slowly increase at first,then rapidly increase ,and then decrease.Here the pH range of both phenolphthalein and methyl orange would fall on this almost vertical portion,and either indicator can be successfully used to find the endpoint.
Does this make sense?