Question

A 100.00 mL sample of hard water is to be titrated with 0.03498 M EDTA solution. A small amount of Mg²⁺ (that required 2.52 mL of EDTA to titrate as a blank solution) is added to the hard water sample. This sample requires 17.89 mL of EDTA to reach the endpoint.

How many moles of Ca²⁺ are in the hard water?

Express answer as a decimal, not an exponent.

Answer 1

mL of EDTA consumed by Ca²⁺ = mL of EDTA consumed by sample - mL of EDTA consumed by blank

                                                    = 17.89 mL - 2.52 mL

                                                    = 15.37 mL

Since 1 L = 1000 mL, 15.37 mL = 0.01537 L.

Moles of EDTA consumed by Ca²⁺ = molarity of EDTA x Volume of EDTA = 0.03498 M x 0.01537 L = 0.0005376 mol

Since EDTA and Ca²⁺ reacts 1:1 mole ratio,

the number of moles of Ca²⁺ in the hard water is 0.0005376 mol.