Question

UV-VIS SPECTROPHOTOMETRY

The mole ratio between a metal , M+ , and a ligand, L, were determined using the continuous variation method. The yellow complex formed absorbs light at 400 nm. Stock solutions containing 1.00 × 10–4 M of each reactants were prepared. Different volumes of the stock solutions were mixed to give 9 different solutions, each having a total volume of 3.00 mL . Table 1 shows the volume of the ligand, L, used and the corresponding absorbances.

Table 1. Absorbance measurements of solutions containing variable volumes of M+ and L

Volume of L, mL	Absorbance at 400 nm
0.3	0.106
0.6	0.214
0.9	0.311
1.2	0.402
1.5	0.442
1.8	0.404
2.1	0.318
2.4	0.222
2.7	0.110

Calculate the mole ratio between the metal and the ligand (using the continuous variation method). Express your mole ratio as M+ : L (ex. 1 M + : 5 L). SHOW ALL STEPS AND CALCULATIONS!!

  

Answer 1

By continous variation method we can find the mole ratio by ploting molefraction of the ligand versus absorbance at 400nm. The maximum (or minimum) on the plot corresponds to the stoichiometry of the two species if sufficiently high concentrations are used.

The mole fraction can be calculated by dividing the volume of ligand by total volume (=3ml).

for eg , at 0.3 ml ligand volume

molefraction = 0.3/3 =0.1

so the table can be reconstructed as

Now plot molefraction vs absorbance

The maximum of the plot corresponds to 0.5 .

So  your mole ratio as M+ : L = 1:1