Question

When you dissolve an eggshell, you might not react it completely. Will the amount of the base used in the titration be higher, lower, or the same as if the reaction went to completion? And will the amount of calcium carbonate calculated in the eggshell be too high, too low, or equal the actual amount?

Answer 1

1) Calcium carbonate is main content of eggshell. On dissolving it may be some part of it will remain undissolved as CaCO3 is stable salt.

2) CaCO3 disolved by following equation,

CaCO₃ -------> Ca²⁺ (aq.) + CO₃^2- (aq.)

I.e. on complete dissociation 1 mole of CaCO3 gives Ca2+ ions.

3) While quatitatively estimating we titrate Ca2+ ions solution with a base like NaOH or KOH.

The pertinant neutralization reaction is

Ca²⁺ + 2 OH^- (aq.) --------> Ca(OH)₂.

3) As condition given that there will be incomplete solubility of CaCO3 it's clear that there will be less Ca2+ ions in solution than expected. And hence less [OH-] will be required than expected (i.e if complete solubility of CaCO3 assumed).

4) Hence accordingly CaCO3 amount calculated will be low as we take amount of CaCO3 equal to Ca2+ ions which is faulty i.e. less than expected.

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