Question

An unknown amount of a compound with a molecular mass of 259.61 g/mol 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 347 nm is 0.525 in a 1.000-cm cuvette. The molar absorptivity for this compound at 347 nm is ε347 = 6557 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?

Answer 1

According to Beer Lamberts law

absorbance = molar absorptivity coefficient x path length of the cuvette x concentration of the sample

path length of the cuvette = 1 cm

molar absorptivity coefficient at 347 nm = 6557 M-1cm-1

a).

Concentration = absorbance / (molar absorptivity coefficient x path length) = 0.525/6557 = 8 x 10^-5 M

note: path length of cuvette is 1 cm

b) Initially 1 mL from the 10 mL flask is diluted to 25 mL volumetric flask. so the dilution factor is 25

thus concentration in the 10 mL flask = 25 x 8 x 10^-5 M = 0.002 M

c). Molarity is the number of moles of a solute in 1L solution. ie 0.002 moles in 1 L (1 L = 1000 mL )

or 0.00002 moles in 10 mL

number moles = mass in g/molar mass

molar mass = 259.61 g/mol

thus mass = 259.61 x 0.00002 = 0.00519 g or 5.196 mg