An introductory Quantitative Laboratory Exercise

Our first year college chemistry course presumes that our students have had previous exposure to elementary laboratory procedures and chemical calculations. In order to rather quickly establish quantitative relationships and to provide an in- teresting laboratory introduction, we have developed an "unknown" which involves quite a few review laboratory techniques and some new mathematical eonceots.

An individual sample vial cou&ining BaClz(H20)2 and NaCl is issued to each student who is asked to both gravimetrically and volumetrically determine the mass of each component present. For a total sample mass in the neighborhood of 4 g careful students can often achieve agreements within f 1 %.

The gravimetric analysis consists of heating the total sample to constant mass and using the mass of water loss to de- termine moles and hence mass af barium chloride dihydrate (BCH) present. The anhydrous sample is made up to a volume of 250.0 ml and aliquot portions are titrated with standardized 0.100 M AgN03 using KzCr01 indicator (Mohr method). The total moles of chloride ion present calculated by this method coupled with the original total sample mass leads to a separate determination of the mass of BCH present. A calculation of this type involving two variables is new for the majority of the students and sets the stage for an early use of the mole method of thought.

A computer print-out available from the instructor lists (1) total sample mass (2) mass of BCH (3) mass of NaCl(4) expected mass H20 loss (5) expected titration value for 10.0 ml aliquot and (6) expected titration value for 25.0 ml aliquot. A check of these results is a big help in pinpointing areas of individual difficulty.

A similar but less mathematically involved unknown is used in our preliminary course. Here we issue an unknown of dried sand and BCH and ask them to determine the mass of sand present by (1) gravimetric determination of BCH, (2) direct filtration and drvine of the sand and (3) volumetric aliquot titration of the chloride ions present. Careful students can get verv mod three-;&checks on these results. . ..

Roth of these laboratory exercises give rather good early experiences with halsnce use, litration techniques, aliquot uw, and mole type calculations. Filtration practice is an ndditional henelit in the earlyexercise. Additional details arenvailnhle upon request.

American River College Saoramento, California 95841

A. J. Crossfield

I90 I Journal of Chemical Education