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JEE Main levelOne or more correct optionsDifficulty 2 of 3eq101

$CaCO X_{3}$ is dissolved by adding a known excess of standard $HCl$ , and the excess acid is titrated with standard $NaOH$ . Which statements are correct?

AOne mole of $CaCO X_{3}$ consumes two moles of $HCl$ .
BA back titration is useful because the sparingly soluble solid reacts slowly with a titrant added dropwise.
CDissolved $CO X_{2}$ should be boiled off before the end point is taken with phenolphthalein, since carbonic acid would otherwise consume $NaOH$ .
DThe moles of $NaOH$ used in the titration equal the moles of $HCl$ consumed by $CaCO X_{3}$ .

Answer: A, B, C

CaCO3 + 2HCl gives CaCl2 + H2O + CO2, so 2 mol HCl per mol; dissolved CO2 (carbonic acid) would affect a phenolphthalein end point, hence boiling. Back titration with excess reagent suits insoluble or slow reactants. NaOH equals the excess HCl, not the HCl consumed by CaCO3, so the third statement is wrong.

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