5 Steps to a 5 AP Chemistry

Experiment 10: Finding the Equilibrium Constant

Synopsis

The value of an equilibrium constant is calculated by measuring (or calculating) the equi-

librium concentrations of the reactants and products. A calibration curve is constructed by

measuring the absorbance of a colored solution versus its concentration. Known quantities

of the reactants are mixed, and the calibration curve is used to determine the concentration

of the colored substance in the resultant solution. (See the Equilibrium chapter.)

Equipment

analytical balance pipet spectrophotometer

test tube(s) or cuvettes volumetric flask

Spectrophotometers

Some of the AP recommended experiments require the use of a spectrophotometer. A

spectrophotometer is an instrument that is used to measure the amount of light absorbed (or

percentage transmitted) by a particular solute in a solution. In order to determine the

absorbance (A) of a sample, the instrument is set to a particular wavelength; a solution, con-

tained in a holder called a cuvette, is placed in a sample chamber and an absorbance reading

is taken. This procedure may be repeated for other solutions or wavelengths. The cuvette is

a standard size to ensure a given path length (b).

A plot of absorbance versus wavelength may be used to identify a component of a solution or

to determine the wavelength of maximum absorbance (maximum molar absorptivity =a). A more

common plot is one of absorbance versus concentration. For this type of plot the instrument is set

at the wavelength of maximum molar absorptivity and the absorbances of solutions of various

known concentrations (c) are measured. This plot should be a straight line. This linear relationship

is called Beer’s law and has the form of A=abc. The concentration of an unknown solution may

be determined by measuring its absorbance and using the plot to find its concentration.

Measurements

Calibration Curve

Quantities of one or more reactants are pipeted into a volumetric flask.

The solutions are diluted to a known volume in the volumetric flask. Measurements of

the absorbance are made with a spectrophotometer.

Equilibrium Concentrations

Different quantities of various reactants are pipeted into a volumetric flask and diluted to

a known volume.

Measurements of the absorbance of these solutions are made with a spectrophotometer.

Calculations

Calibration Curve

The concentration of the absorbing species is calculated using the initial pipeted volumes

and the final volumetric flask volume. These concentrations are plotted versus the absorbance

of the solution.

Equilibrium Concentrations

The final concentration of each reactant is calculated from the final volume and the volume

and concentration of the solution pipeted into the volumetric flask. The calibration curve

is used to find the equilibrium concentration. Using the balanced chemical equation, the

equilibrium concentrations of the other substances may be calculated.

Experimental  289