The stoichiometry of a metal–ligand complex, MLn, was determined by the method o

Question

The stoichiometry of a metal–ligand complex, MLn, was determined by the method of continuous variations. A series of solutions was prepared in which the combined concentrations of M and L were held constant at 5.15 10–4 M. The absorbances of these solutions were measured at a wavelength where only the metal–ligand complex absorbs. Using the following data, determine the formula of the metal–ligand complex.

mole fraction M mole fraction L absorbance 1.0 0.0 0.001 0.9 0.1 0.126 0.8 0.2 0.260 0.7 0.3 0.389 0.6 0.4 0.515 0.5 0.5 0.642 0.4 0.6 0.775 0.3 0.7 0.771 0.2 0.8 0.513 0.1 0.9 0.253 0.0 1.0 0.0

Explanation / Answer

The reaction is

M + nL = MLn                      (ignore charges)

The maximum concentration (indicated by the maximum absorbance) of MLn is obtained

When

Molar fraction of L was 0.6…0.7 and

Molar fraction of M was 0.3…0.4

The ratio of these two value is a ratio of integers, so

The best fit is   0.66/0.33 = 2/1

The formula is ML2 .

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