You dissolve a compound with a molecular mass of 267.84 in a 10-mL volumetric fl

Question

You dissolve a compound with a molecular mass of 267.84 in a 10-mL volumetric flask. You remove a 1.00-mL aliquot, place it in a 25-mL volumetric flask and dilute it to the mark. The absorbance of this diluted solution at 335 nm was 0.542 in a 1.000-cm cuvet. The molar absorptivity for this compound at 335 nm is 335 = 6539 M–1 cm–1.

(a) What is the concentration of the compound in the cuvet?

(b) What is the concentration of the compound in the 10-mL flask?

(c) How many milligrams of compound were used to make the 10-mL solution?

Explanation / Answer

Applying Lambert-Beer’s law the concentration of a solution can be obtained.

A = dC

Where:

A = Absorbance = 0.542

= Molar absorptivity (L mol-1 cm-1) = 6539 M–1 cm–1

d = Path length of the cuvette containing the sample (cm) = 1.000-cm

C = Concentration of the compound in the solution (mol L-1) =?

Therefore,

C = A/d = 0.542/6539/1 = 0.000083 M

(a) concentration of the compound in the cuvet = 0.000083 M

(b)10 mL --> 1 mL --> 25 mL, so the dilution factor = 25. Therefore the concentration of the compound in the 10-mL flask = 25*0.000083 = 0.0020 M

(c) Now, 10 mL 0.0020 M solution 10*0.0020/1000 moles = 0.00002 moles.

Molar mass = 267.84 g/mol.

Therefore, amount of sample = 267.84*0.00002 = 0.00536 g = 5.36 mg were used to make the 10-mL solution.

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