[muzza001]

The calcium hardness of a water sample is found by complexation of Ca2+ with excess EDTA and back-titrating the excess with MgCl2.

A known amount of EDTA (0.0100 M, 25.00 mL) is added to a 20 mL aliquot of the water sample. (Y is used to symbolise the EDTA molecule)

Ca2+(aq) + Y4-(aq) <-> CaY2-(aq)

This represents an excess of EDTA so all of the Ca2+ is complexed. The excess EDTA in the mixture is determined by titration with standard 0.0100 M MgCl2.

Mg2+(aq) + Y4-(aq) <-> MgY2-(aq)

Since the calcium complex is more stable than the magnesium complex, the MgCl2 will only react with the excess EDTA and will not displace complexed EDTA from the Ca2+. Thus, the amount of EDTA reacting with Ca2+ in the water sample is the difference between the starting amount of EDTA and the excess EDTA.

Eriochrome Black T is used as the indicator and turns purple indicating the equivalence point when 20.2 mL of MgCl2 has been added.

Q. What is the ppm CaCO3 hardness of the water sample?


Any calculations shown for this question would be a great help, many thanks.

[Unknown008]

Using the volume of MgCl2 used and its concentration, can you find the number of moles of excess EDTA?

When you have the number of moles of excess EDTA, find the initial moles of EDTA used. The difference gives the number of moles of EDTA which reacted with Ca ions. Now that you have the number of moles of Ca^2+ and the volume of water, can you find the concentration of CaCO3.

Once you get that, it's only a matter of conversion of concentration from M to ppm.

Post what you get and show your work. If you get stuck, show what you would have done. It doesn't matter if it's wrong. You have to learn, and if you already went through it once and understand the various problems you meet, you will be gaining much more :)