5 Steps to a 5 AP Chemistry 2019

Experimental Investigations ❮ 287

number of measurements at different concentrations, you could create a graphical relation-

ship between the amount of light absorbed and the concentration of the solution. By using

this relationship, you could determine the concentration of an unknown solution.

Equipment

Spectrophotometer (commonly SPEC 20)

Cuvettes (sample tubes for the spectrophotometer)

Stock solutions (of known concentrations) of the solute (commonly some dye)

Solution of unknown concentration (may be a household substance)

Assorted glassware, including volumetric glassware

Measurements

The student will make several dilutions of the stock solution and will calculate the concen-

tration of each dilution (using M 1 V 1 = M 2 V 2 ). The transmittance (%T) will be measured

for each solution (remembering to subtract the transmittance of the solvent—this may be

done by adjusting the spectrophotometer to 100% T and then measuring the transmittance

of the solution).

Calculations

To determine the relationship between the concentration of the solution and the transmit-

tance, plot the molarity of the different solutions versus the transmittance (expressed as a

decimal). The absorbance (Abs) of the solution (how much light is absorbed) is calculated

by the formula Abs = - log (T), where T is the transmittance of the solution (not the per-

cent transmittance). On a SPEC 20 you can read absorbance directly.

Comments

If you are asked for the mass of the solute in the unknown, you first determine its molar

concentration using your spectroscopy data. Then, using the molar concentration, the

volume of the solution, and the molar mass of the solute, you can calculate the grams of

solute present in the sample.

Experiment 2: Spectrophotometry

Synopsis

Specific experiments that are performed in this investigation use the concepts and tech-

niques developed in Experiment 1: Spectroscopy in order to determine the mass percentage

of a particular substance in a solid sample. Determination of the amount of copper in a

brass sample is a common experiment that is used in this category as well as the amount

of iron in a vitamin pill. First, the “best” wavelength to be used is determined. The “best”

wavelength is the one that gives the maximum absorbance of the chemical species being

determined. Next, solutions of the solute being determined are prepared and their absor-

bance is measured using a spectrophotometer. A plot of absorbance versus concentration

(Beer’s law) is prepared. The solid sample is dissolved and diluted to a certain volume. The

absorbance of a portion of this sample is measured and its concentration is determined

using the graph. From this information the mass of the substance can be found. Using this

mass information and the mass of the sample allows you to calculate the mass percentage

of the substance in the sample.