To determine either the equivalent weight of a commercial lime sample or the percentage of CaO in a sample.
Review the discussion on equivalent weights in the laboratory exercise entitled "Equivalent Weight of an Unknown Acid".
At this point we have done enough titration labs that you should be able to create your own procedure. Commercial lime is usually a mixture of CaO and Ca(OH)2. You will need to make some assumptions about its composition and do some preliminary calculations before deciding on a procedure to follow. Good Luck!
1. Calculate either the equivalent weight of your commercial lime sample or the percentage of CaO in your lime sample.
2. How would your answer in question 1 be affected if the sample was inadvertently contaminated with an inert substance like sand by your lab partner after massing? Explain.
3. From the class values listed on the chalkboard for question 1 determine which ones would be considered outliers at the 96% confidence level and rejected. After rejecting these values calculate the class average and the standard deviation.
4. Summarize all calculations in a nice neat table.
I have found that one of the best ways to teach about back titrations is by experience. This laboratory investigation not only gives students a chance to analyze a commercial product, but to see that a standard "straight-forward-titration" will not work because of the low reaction rate of the components of lime.
The lab is written so that students must give some thought as to the procedure, which I generally have to help them with, because they are so use to following canned experiments. We discuss (1) the amount of lime to be analyzed; (2) the concentration of acid (their choice of nitric or hydrochloric); and (3) standardization of the acid. I then solicit volunteers to mix up and standardize the acid. The class then proceeds with the titration. They soon find out that the endpoint is not sharp and well defined.
This then is an excellent opportunity to talk about back titrations which will mean more now because students have observed first-hand the problems associated with a "normal titration". We then discuss procedural modifications that will take care of the poor endpoint. Students then repeat the lab.
A lot can be taught with this laboratory investigation. If time permits you can "lead" them to choose sulfuric acid the first time around. This of course will be a disaster because of the insoluble product calcium sulfate.