Question

An EDTA solution was prepared by dissolving the disodium salt in 1l of water. It was standardized using 0.5063 gram of primary standard CaCO3 and consumed 28.50 ml of the solution. The standard solution was used to determine the hardness of a 2L sample of mineral water, which required 35.57 ml of EDTA solution. Express the analysis in terms of ppm CaCO3 ? Answers. 316 ppm

Answer 1

The reaction for the standardization of EDTA disodium salt solution using CaCO3 is:

Na2EDTA + CaCO3      Na2CO3 + CaEDTA

Mass of CaCO3 = 0.5063 gram = 506.3 mg

Molar mass of CaCO3 = 100.0869 g/mole

Moles of CaCO3 = 0.5063 / 100.0869

= 0.00506 moles

= 5.06 milimoles

From the equation it is clear that,

milimoles of EDTA = milimoles of CaCO3

So, milimoles of EDTA = 5.06

Concentration of EDTA = 5.06 / 28.50

= 0.1775 M

Now,

Volume of EDTA required to determine the hardness of a 2L sample of mineral water = 35.57 mL

Milimoles of EDTA used = 0.1775 * 35.57

= 6.313 milimoles

If the standardized EDTA disodium salt solution is used to determine the hardness of mineral water, then the amount of CaCO₃ is:

Milimoles of CaCO3 = milimoles of EDTA

So, milimoles of CaCO3 = 6.313

Mass of CaCO3 = 6.313 * 100.0869

= 631.84 mg

Therefore, the amount of CaCO₃ in ppm is

ppm CaCO3 = 631.84 mg / 2L

= 315.92

316 ppm

Hence,

Amount of CaCO3 = 316 ppm