Question

A 35 mL sample of water is triated with 0.0100 M EDTA. Exactly 9.70 mL of EDTA are required to reach the EBT endpoint. Calculate the total hardness in ppm CaCO3.

Answer 1

Hardness in water is due to the presence of Ca²⁺ ions. These ions can be analysed by titration with EDTA(ethylene diamine tetraacetic acid).

The reaction goes like this:

H₂Y^2- + Ca²⁺ ------> CaY^2- + 2H⁺

molar ratio is 1:1 (stoichimetric numbers ratio)

Given data: 0.01M EDTA => 0.01 mol EDTA per litre of solution

Volume of EDTA required = 9.7 ml = 0.0097 lt

no. of moles of EDTA required = 0.01 mol EDTA per litre of solution *  0.0097 lt = 9.7*10^-6 mol

Since the molar ratio is 1:1 the no. of moles of Ca²⁺ ions in the sample = 9.7*10^-6 mol

Now for converting to ppm CaCO₃:

Molecular weight of CaCO₃ = 100g/mol

Therefore in the water sample weight of equivalent CaCO₃ = 100*9.7*10^-6 = 9.7*10^-4g = 0.97mg

ppm CaCO₃ = mg CaCO₃ in 1 litre of solution

Here the volume os sample solution = 35ml = 0.035 lt

Therefore hardness of the sample = 0.97/0.035 =27.7 ppm