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-Lambert 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 three equilibrium solutions of Ag_2CrO_4 at the same wavelength. Plot a calibration graph using the graph paper provided. Demonstrate how you would use the calibration curve to determine the chromate (CrO_4^2) concentrations of equilibrium solutions, complete Table B of the table and determine the Ksp in each case. Ag_2CrO_4 (s) 2Ag^+ (aq) + CrO_4^2- (aq) DATA AND CALCULATIONS

Explanation / Answer

Calculation : concentration of [CrO4^2-] in the final solution

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

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

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

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

molar absorptivity of CrO4^2- at 375 nm = 4820 M-1.cm-1 (from literature)

From absorbance,

[CrO4^2-] in solution = absorbance/molar absorptivity

therefore,

Solution 1, Equilibrium [CrO4^2-] = 0.038/4820 = 7.88 x 10^-6 M

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

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

Solution 4, Equilibrium [CrO4^2-] = 0.602/4820 = 1.25 x 10^-4 M

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

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

So,

Ksp for,

Solution 1, Ksp = (2 x 7.88 x 10^-6)^2.(7.88 x 10^-6) = 1.96 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 1.25 x 10^-4)^2.(1.25 x 10^-4) = 7.81 x 10^-12

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