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NEET levelSingle correct optionDifficulty 1 of 3eq038

A solution of pure $Na X_{2} CO X_{3}$ is titrated with standard $HCl$ . If the phenolphthalein end point is reached after adding $V_{1}$ mL, the methyl orange end point (total volume from the start) is reached after:

A $2 V_{1}$ mL
B $V_{1}$ mL
C $V_{1} /2$ mL
D $3 V_{1}$ mL

Answer: A

Phenolphthalein marks

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

(one

H X^{+}

per carbonate) and methyl orange marks

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

(a second, equal amount). The total volume is therefore

2 V_{1}

.

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