Answer to Question #208783 in General Chemistry for KPOPOII

Question #208783

A 20.0 mL portion of a deep well water sample was diluted to 250.00 mL solution. A 50.0 mL aliquot of this diluted sample water was buffered at pH 10 and required 8.50 mL of a standard EDTA. The titrant was standardized using 100.0 mL containing 0.8622 mg primary standard calcium carbonate, CaCO₃ (FW=100.09 g/mol) which required 15.45 mL for titration.

(a) Identify a possible indicator and its endpoint color.

(b) Calculate the molarity of the EDTA solution.

(c) Calculate the total hardness of the water as ppm CaCO₃.

(d) Identify the quality of the water sample in terms of hardness.

Expert's answer

Answer a)

The titration involved is a complexometric titration between EDTA and Ca2+ at pH 10.

The indicator used is Eriochrome Black T (EBT) which reacts with Ca2+ to form red colored complex. When there is slight excess of EDTA, EBT is displaced. The free indicator is wine red in color.

So, the end point is appearance of wine red color.

Answer b)

Hence, concentration of EDTA is 5.575 x 10-4 mol/L.

Answer c)

Concentration of CaCO3 in well water is 118.6 ppm

Answer d)

Water is considered hard if it contains CaCO3 in range of 100-200 ppm.

As the calculated hardness is 118.6 ppm which lies in given range, so, well water is considered hard.

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