Question

A 0.2054 g sample of CaCO3 (primary standard) is dissolved in hydrochloric acid and the solution is diluted with water to 250.0 mL (solution A). A 50.0 mL aliquot of solution A is titrated with 41.12 mL of EDTA solution. a) I calculated a Molarity of 0.00998. Is this correct? (show all calculation). b) Now a 100.0 mL sample of water containing Ca2+ and Mg2+ is titrated with 22.74 mL of the EDTA solution in the example above at pH = 10.00. Another 100 mL sample is treated with NaOH to precipitate Mg(OH)2, and then titrated at pH 13 with 15.86 mL of the same EDTA solution. Calculate the ppm of CaCo3 and MgCO3 in the sample.

Answer 1

Ca2+ + EDTA4- CaEDTA2-(the complexation reaction)

1) primary std

moles of CaCO3=0.2054g/100.0 g/mol=0.002054 moles

Molarity of CaCO3=moles/volume of solution= (0.002054 moles/250) * 1000 L =0.008216 moles/L

standardization of EDTA

volume of EDTA=41.2ml

volume of Std CaCO3=50 ml(sol A)

Molarity of EDTA=Molarity of CaCO3 * volume of Std CaCO3/volume of EDTA

                      =0.008216 moles/L * 50ml/41.2ml=0.00998 moles/L

2)volume of hard water(containing Ca and Mg ions)=100 ml

volume of EDTA=22.74 ml

Molarity of CaCO3 in the water sample=Molarity of EDTA * volume of EDTA/volume of water sample

                                                        =0.00998 moles/L*22.74 ml/100ml=0.00227 moles/L

Mg2+ + EDTA4-MgEDTA2- the stability constant of calcium complex is higher than magnesium complex ,so first all the calcium forms complex.

next titration of MgCO3 with EDTA(after precipitation of Mg(OH)2)

3) Similarly,

volume of hard water(containing Mg ions)=100 ml

volume of EDTA=15.86 ml

Molarity of CaCO3 in the water sample=Molarity of EDTA * volume of EDTA/volume of water sample

                                                        =0.00998 moles/L*15.86 ml/100ml=0.00158 moles/L

4) ppm of CaCO3 (mg/L)

Molarity of CaCO3 in water=0.00227 moles/L

mass of CaCO3 in water=0.00227 moles/L* 100g/mol=0.227 g/L=0.227*1000mg/L=227 mg/L=227 ppm

5)ppm of MgCO3 (mg/L)

Molarity of MgCO3 in water=0.00158 moles/L moles/L

mass of MgCO3 in water=0.00158 moles/L* 84.31g/mol=0.1332 g/L=0.1332*1000mg/L=133 mg/L=133 ppm