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A local beverage company is trying to find a new way to soften the hard water that is supplied to the company's factories. The process they are using now makes the water safe to drink but it is also expensive. While other groups are assigned to test the safeness of softened water, your group has been assigned to test the actual water softening process. Your group will use an ion-exchange process to soften the hard water samples. The equation for the ion-exchange that results in softened water is as follows:

2NaR(s) + Ca2 + CaR2(s) + 2Na+(aq)

The R- in the equation above represents the anion of exchange. When hard water containing Ca2+ ion comes in contact with the NaR, the Na+ ion is replaced by the Ca2+ ion.

Keep in mind the following:
Hardness Range(ppm Ca) Hardness Description

0-50 Soft

51-50 Moderately Hard

151-300 Hard

>300 Very Hard


Using the following method hard water can be softened:
First do an EDTA titration of a water sample to determine the hardness of the sample. Then in a buret place cotton followed by a measurement of Rexyn. Pour a new 25ml of the water sample into the buret. Open the stopcock enough to allow the water to drop through at a considerably slow pace. Collect the softened water in a beaker. In order to determine the difference in the hardness of the water do another EDTA titration of the softened water. Repeat this several times to determine the least amount of Rexyn that can be used to bring the water into the range of 0-50ppm Ca.


Pre Lab Questions:

1. What method will you use to determine the least amount of Rexyn that can be used to soften the water?

2. Rexyn is in the form of small round beads, so why is it important to put cotton in the buret before pouring the Rexyn in?


Post Lab Questions:

1. What was the hardness of the water before softening?

2. Report the amount of Rexyn used for each trial and the hardness of the water after.

3. What is the least amount of Rexyn that can be used to get a value where the water can be considered soft?