Experiment 10 Finding the Equilibrium Constant

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. (See the Equilibrium chapter.) 

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. 

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. 

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