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# Experiment 10: Finding the Equilibrium Constant for AP Chemistry

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By McGraw-Hill Professional
Updated on Feb 9, 2011

For a quick review on equilibrium, refer to the following concepts:

### Synopsis

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

### 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, contained 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.

The equilibrium concentrations are inserted into the reaction quotient expression, and the equilibrium constant is calculated.

### Comments

Any equilibrium may be used, as long as one substance is colored.

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