294 Step 4. Review the Knowledge You Need to Score High
The mass of the substance of interest divided by the mass of the sample and then mul-
tiplied by 100% gives the percent of a substance in the sample.Comments
Common precipitates used include AgCl and BaSO 4.Experiment 17: Colorimetric Analysis
Synopsis
This experiment involves determining the amount of a substance in a solution. A calibra-
tion curve is constructed plotting the measured absorbance versus the concentration of a
colored substance. The concentration of an unknown solution may be determined by
reversing this process. (See the Stoichiometry chapter.)Equipment
buret pipet spectrophotometer
support stand clamp test tube(s) or cuvettesMeasurements
Using either a pipet or a buret, quantities of standard solutions are measured. (If a buret is
used, separate measurements of the initial and final volumes are needed.) Solvent may
be added to dilute the samples if needed. These are the known solutions from which
a calibration curve will be constructed.
The absorbance of each solution is determined with a spectrophotometer.
The absorbance of one or more unknown solutions is determined with a spectro-
photometer.Calculations
Concentrations of the known solutions are calculated using the dilution equation.
A plot of absorbance versus concentration for the known solutions is made.
The plot allows the absorbance of the unknown solution(s) to be converted to
concentration.Comments
Any colored substance may be used.Experiment 18: Chromatographic Separation
Synopsis
Two or more substances are separated by the differences in their affinity to paper or some
other material. This affinity is related to the intermolecular forces between the substances.
(See the chapter on Solids, Liquids, and Intermolecular Forces.)Equipment
beaker(s) capillary tubes
pipet test tube(s)
ion-exchange resin or silica gel chromatography paper (filter paper)