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Question

500 mL of a solution of CaCO3 and NaOH was first titrated with N10 HCl using phenolphthalein indicator. 18.5 mL of HCl was required for the end point. After that methyl orange indicator was used and 2.5 mL of same HCl was required for the next end point. Find out the mass of CaCO3 in the mixture.

A
0.25 g
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B
0.025 g
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C
0.057 g
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D
0.875 g
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Solution

The correct option is B 0.025 g
Phenolphthalein will only neutralise 50% of CaCO3.
12 Milliequivalents of CaCO3+Milliequivalents of NaOH=Milliequivalents of HCl=110×18.5=1.85

The methyl orange indicator will neutralise the rest 50% of CaCO3.
12 Milliequivalents of CaCO3=Milliequivalents of HCl=110×2.5=0.25
Milliequivalents of CaCO3=0.25×2=0.50
WCaCO3MCaCO3×2×1000=0.5
WCaCO3=0.5×1002×1000=0.025 g

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