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

Question

A spectrophotometer was used to obtain absorbance values four (4) standard chromate of (CrO_4^2-) solutions at 375 nm. The standard solutions were prepare 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 describe 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 doubleheadarrow 2 Ag+(aq) + CrO_4^2+ (aq) DATA AND CALCULATIONS

Explanation / Answer

Ans. Part A. Let’s use first data (tube 1) of the table.

Given,

            Volume of standard K2CrO4 taken, V1 = 2.0 mL

            Final volume made upto, V2 = 100.0 mL

            Concentration of standard K2CrO4, C1 = 0.0012 M

Now, Using                            C1V1 = C2V2                        - equation 1

C1= Concentration, and V1= volume of std. solution 1     ;(0.0012 M K2CrO4)

C2= Concentration, and V2 = Vol. of final solution 2                 ;( Diluted soln. in tube 1)

Putting the values in equation 1-

            0.0012 M x 2.0 mL = C2 x 100.0 mL

            Or, C2 = (0.0012 M x 2.0 mL) / 100.0 mL = 0.000024 M

Therefore, in tube 1, final [K2CrO4] = 0.000024 M

Part B.            Absorbance, A = a b c                     - equation 2

Putting the values in equation 2-

            0.038 = a x 1.0 cm x 0.000024 M

            Or, a = 0.038 / (0.000024 M cm)

            Or, a = 1583.33 M-1 cm-1

Therefore, molar absorptivity of K2CrO4 in the given case = 1583.33 M-1 cm-1

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