Percent water in green crystalline compound: An inquiry lab

(This is the second article in a four-part series.)

Our previous article¹ detailed the conversion of a synthesis laboratory activity from a structured inquiry format to guided inquiry. In the guided inquiry version of the lab, students create and execute a procedure to produce approximately three grams of an iron oxalate complex² having the general formula K_x[Fe(C₂O₄)_y]•zH₂O. With this experiment, students begin a guided inquiry that ultimately provides values for x, y, and z in the general formula. Determination of the three variables constitutes a complete analysis of the iron oxalate complex.

The focus of this article is the gravimetric analysis which yields a value for “z” in the complete analysis. We converted a structured inquiry into a guided inquiry, in which students generate notes for a workable procedure by means of a series of pre-lab questions, followed by a pre-lab discussion. The pre-lab discussion was designed around eight questions derived from the original steps in the structured inquiry version of the lab. After the pre-lab discussion is concluded, students are directed to “[d]esign an experiment to determine the percent water of hydration in K_x[Fe(C₂O₄)_y]•zH₂O.”³ Students were required to show the procedure to the instructor for approval before performing the lab. Also, students are asked to submit a data table consistent with their procedure. Such a table would undoubtedly account for the masses of the empty crucibles, the crucibles and the hydrate samples, and the crucibles with the anhydrous complex for all planned trials.

To introduce concepts and begin to guide students to a workable procedure, pre-lab questions used data sets:

Properties of Five Compounds

Pre-Lab Question: Which of these compounds are hydrates? How did you know?

Expected Response: Ni(NO₃)₂•6H₂O, BaCl₂•2H₂O and Na₂CO₃•10H₂O. These compounds contain water as part of the formula.

Students could arrive at the correct answer either by recognizing the dot notation, or from the molar mass data.

Pre-Lab Problem 2:  A 1.152-gram sample of a hydrate compound was heated in an oven at 110oC. Data for the sample’s mass were recorded at several time intervals, as the table below shows.

a.   At what time is it no longer necessary to continue heating? Explain.

Expected Response: 2.5 hours. After 2.5 hours the hydrate compound no longer loses water.

b.   What was the mass percent of water in the original sample?

Expected Answer: (0.428 g / 1.152 g) x 100 = 37.2%
The data set strongly suggests a way that students can determine how long to heat their samples.

After completion of the pre-lab questions, the instructor leads a discussion in which students generate additional details important to a workable procedure. The discussion, designed to take one class period, introduces a list of available equipment: oven (set at 110^oC to 120^oC), milligram balance, porcelain crucibles, crucible tongs, 50-mL beakers, and beaker tongs. Then, students respond to questions⁴ that focus on the equipment in the list. The questions are intended to give students awareness of factors that a research scientist would consider when developing a new experimental procedure. The eight questions are:

1. What measurements would be needed to calculate the percentage H₂O?
2. Why might you use more than one crucible?
3. Why handle the crucible with tongs?
4. Why heat the empty crucibles?
5. What mass of green crystal compound should be used in each crucible?
6. What benefit is there to placing crucibles in 50-mL beakers?
7. We suggest 2-hr heating time. Why might we limit the time to 2 hr?
8. What advantage(s) might be realized from multiple heatings?

The questions encourage good lab practice through equipment selection and handling, seeking reproducibility of results through multiple trials, isolation of variables, and safety. Each question originated from a step in the old structured inquiry version of the lab. For example, question 5 above, which brings sample size into the discussion, was represented in step 5 of the structured inquiry procedure⁵ by

5.   Transfer between 1.000 and 1.200 grams of the green crystalline compound into each crucible. Mass the crucibles and crystals, and use the same balance you used in measuring the masses of the empty crucibles.

Unlike the structured inquiry step shown, the guided inquiry question gives students an opportunity to discover the two main factors that determine the mass of hydrate to use in the procedure. Students will need to reserve a portion of the original sample for use in the two subsequent procedures for the total analysis. Also, students become aware of the need for large sample sizes in gravimetric analysis. Because the calculations involve differences in mass measurements, the original samples must be fairly large, or there is a risk of losing significant figures in the result.

Beginning in 2013, the AP Chemistry Course Audit⁶ requires course syllabi to explicitly list the Science Practices for all labs. Science Practices⁷ 2.1, 2.2, 4.1, 4.2, 4.3, and 5.2 are present in this lab.

The next installment in this series will explore conversion of the lab to determine the percent potassium and percent iron by means of an acid-base titration following a quantitative cation exchange.

References

1. April 2013, Chem 13 News, pages 8-9.
2. CAS Registry No. 5936-11-8.
3. GCII: what percent of the green crystalline compound is water? Southeast Missouri State University, Cape Girardeau MO 63701.
4. Developing a Procedure for the Green Crystal II Lab. Southeast Missouri State University, Cape Girardeau MO 63701.
5. Green crystalline compound part ii: Determination of percent water. Southeast Missouri State University, Cape Girardeau MO 63701.
6. http://www.collegeboard.com/html/apcourseaudit/