[Aakash]

In the neutralization of Na₂CO₃ with HCl we use phenolpthalein for the first end point
at which NaHCO₃ + NaCl is produced.
My question is that why is it that phenolpthalein turns transparent at the first end point when NaHCO₃ is also a base.

[Hunter2]

It is matter of pH Phenolphthaleine change at pH 8.2 the colour. At that pH HCO₃^- is still existing. You need another indicator like Methylorange.

[Borek]

See related discussion of the Warders titration:

http://www.titrations.info/acid-base-titration-sodium-hydroxide-and-carbonate

Note NaOH gets neutralized first, so the curve observed doesn't differ much from the curve for pure Na₂CO₃.