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NEET levelSingle correct optionDifficulty 1 of 3eq115

In the titration of $Na X_{2} CO X_{3}$ solution against standard HCl, the colour change of phenolphthalein (pink to colourless) marks the stage at which the carbonate has been converted into:

A $NaHCO X_{3}$ (with NaCl)
B $H X_{2} CO X_{3}$ i.e. $CO X_{2}$ (with NaCl)
C $NaOH$ (with NaCl)
D $Na X_{2} CO X_{3}$ is still unreacted and only NaCl has formed

Answer: A

Phenolphthalein changes colour near pH 8.3, where

CO X_{3} X^{2 -} + H X^{+} HCO X_{3} X^{-}

is just complete. Hence the phenolphthalein end point corresponds to

NaHCO X_{3}

; the second stage needs methyl orange.

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