Question

19) Water from University Lake was analyzed for its Fe content. A 25.0-mL sample of lake water was acidified with nitric acid and treated with excess KSCN to form a red complex. (KSCN itself is colorless.) The solution was then diluted to 100.0 mL and put in a 1.00 cm pathlength cell, where it yielded an absorbance of 0.545. For comparison, a 10.0-mL reference sample of 6.80 x 10 M Fe was treated with HNO3 and KSCN and diluted to 50.0 mL. The reference solution was also placed in a 1.00-cm cell and gave an absorbance of 0.312, What is the concentration of Fe3+ in University Lake? A) 2.79 x 103 M B) 749 x 104 M C 1.23 x 103 M D) 4.52 x 104 NM E) 5.79 10-3 M F) 6.62x 104 M G) 5.36 x 104 M 9.50 x 10 M -4 312

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Answer 1

The relation ship between absorbance (A )and concentration (c) is as follows Aab Here, a is the molar absorptivity and b is the pathlength At constant a and b; 4 is the absorbance at concentration ci A, is the absorbance at concentration c Absorbance of sample(A) is 0.545 Concentration of reference Fe+ is c Absorbance ofreference(42) is 0.312. 6.80x10 M) (10.0 mL) 50.0 mL Concentration of reference Fe (c) - 1.36x10* M 0.312 1.36x104 M 0.545 0.545 10.312 1.36x104 M)- 2.376x10-4 M Concentrtion of Fe in University lake . (2376 10 )(100 maL) 9.50x10'M Concentration of Fe in University lake - 25.0 mL