Question

1. A series of solutions, containing cobalt (I1) chloride were prepared with concentrations shown below. Using PhET Simulation find the absorbance values corresponding to each solution at the preset value of wave length 549 nm, using 1 cm wide cuvette. Concentration of Co (II), mM 121 98.0 60.0 24.0 15.0 Absorbance Plot a calibration line Absorbance versus Concentration. From the graph determine the specific absorptivity. 2. Using the data you obtained from the graph in the previous problem determine the concentration of cobalt (II) in 1.975 g sample of a substance, which was dissolved in water in a 100 mL volumetric flask and then diluted to the mark. For the absorbance of the cobalt solution from the flask a value of 0.55 was obtained. What is the percentage of cobalt in the unknown compound? Atomic mass of Co is 58.933.

Answer 1

Calculate the molar absorptivity (epsilon) A = 0.0072 [CO ()] - 0.0001 slope = epsilon x b = 0.0072 1/m M b = path length of light = 1 cm epsilon times 1 cm = 0.0072 1/m M epsilon = 0.0072 1/m M/1 cm epsilon = 0.0072 1/m M cm epsilon = 0.0072 1/m M cm x 1000m M/1 M epsilon = 7.2 1/M cm calculate the specific absorptivity (k) k = 7.2 1/M cm x 1 mol co (II)/58.9339 CO (II) = 7.2 l/MOL CM X 1 mol co (II)/58.933 g co (II) K = 0.122 L/g cm (2) calculate the [co (II)] in the sample solution A = 0.0072 [CO (II)] - 0.000) 0.55 = 0.0072 [CO(II)] - 0.0001 0.55 + 0.0001/0.0072 = [CO (II)] 76.4 m M = [CO (II)] [CO (II)] = 76.4 m M times 1 M/1000m M = 0.0764 M.= 0.0764 M Calculate the moles of (II) in the solution sample moles co (II) = 100 m L times 0.0764 mol co (II)/1000m L.