An unknown amount of a compound with a molecular mass of 287.90 g/rnol is dissol

Question

An unknown amount of a compound with a molecular mass of 287.90 g/rnol is dissolved in a 10-mL volumetric flask. A 1.00-mL aliquot of this solution is transferred to a 25-mL volumetric flask and enough water is added to dilute to the mark. The absorbance of this diluted solution at 331 nm is 0.550 in a 1.000-cm cuvette. The molar absorptivity for this compound at 331 nm is E_331 = 5663 M^-1 cm^-1. (a) What is the concentration of the compound in the cuvette? (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

A = cl

A = 0.550

= 5663 M-1cm-1

l = 1 cm

c = A/l = 0.550 /(5663 M-1cm-1 x1 cm) = 9.71x 10-5 M

The concentration of the compound in the cuvette = 9.71x 10-5 M

b. compound (taken in 10 mL) -----> 1mL was taken --------> diluted to 25 mL -----> taken in cuvette

The concentration of the compound in 10 mL flask = 9.71x 10-5 M x 25x 10 = 0.0242 M (Dilaution factor = 25 x 10)

c. Molecular weight of the compound is 287.90 g/mol

Amount present in 10 mL solution =  0.0242 M =  0.0242 mol/L x 287.90 g/L = 6.990 g/ L

= (6.990 g/100) = 69.9 mg (amount present in 10 mL = 1000/10 = 100)

Amount present in 10 mL solution = 69.9 mg

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