A spectrophotometer was used to obtain absorbance values of four (4) standard ch

Question

A spectrophotometer was used to obtain absorbance values of four (4) standard chromate (CrO_4^2) solutions at 375 nm. The standard solutions were prepared by diluting various volumes of 0.0012 M potassium chromate K_2CrO_4. Complete Table A below. The relationship between absorbance and concentration is defined by the Beer Law described below. A = abc, where A = absorbance a = molar absorptivity b = path length, 1 cm c = concentration (M) Absorbance values were also obtained for two equilibrium solutions of AgrCrO_4 at the same wavelength. Plot a calibration graph using the graph paper provided. Demonstrate how you would determine the chromate (CrO_4^2) concentrations of equilibrium solutions using the calibration graph, complete Table B and determine the Ksp in each case. Ag_3CrO_4(s) 2Ag^+(aq) + CrO_4^2-(aq) DATA AND CALCULATIONS

Explanation / Answer

Calculations

From stock solution dilution

Final molar concentration of [CrO4^2-] in solution = molarity of stock solution x volume of stock solution/final volume

So,

Solution 1, [CrO4^2-] = 0.0012 M x 2 ml/100 ml = 2.4 x 10^-5 M

Solution 2, [CrO4^2-] = 0.0012 M x 10 ml/100 ml = 1.2 x 10^-4 M

Solution 3, [CrO4^2-] = 0.0012 M x 20 ml/100 ml = 2.4 x 10^-4 M

Solution 4, [CrO4^2-] = 0.0012 M x 30 ml/100 ml = 3.6 x 10^-4 M

Calculations from absorbance values,

Equilibrium concentration [CrO4^2-] = absorbance/molar absorptivity

molar absorptivity [CrO4^2-] = 4820 M-1.cm-1

So,

Solution 1, Equilibrium concentration [CrO4^2-] = 0.038/4820 = 7.89 x 10^-6 M

Solution 2, Equilibrium concentration [CrO4^2-] = 0.185/4820 = 3.84 x 10^-5 M

Solution 3, Equilibrium concentration [CrO4^2-] = 0.400/4820 = 8.30 x 10^-5 M

Solution 4, Equilibrium concentration [CrO4^2-] = 0.502/4820 = 1.04 x 10^-4 M

Thus,

Ksp = [Ag+]^2.[CrO4^2-]

[Ag+] = 2 [CrO4^2-]

So,

Solution 1, Ksp = (2 x 7.89 x 10^-6)^2.(7.89 x 10^-6) = 1.97 x 10^-15

Solution 2, Ksp = (2 x 3.84 x 10^-5)^2.(3.84 x 10^-5) = 2.26 x 10^-13

Solution 3, Ksp = (2 x 8.30 x 10^-5)^2.(8.30 x 10^-5) = 2.29 x 10^-12

Solution 4, Ksp = (2 x 1.04 x 10^-4)^2.(1.04 x 10^-4) = 4.50 x 10^-12

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