An unknown amount of a compound with a molecular mass of 260.73 g/mol is dissolv

Question

An unknown amount of a compound with a molecular mass of 260.73 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 353 nm is 0.565 in a 1.000-cm cuvette. The molar absorptivity for this compound at 353 nm is 353 = 5793 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

the molecular formula = 260.73 g / mole

a) we will use beer lambert's law here

Absorbance = X c X l

c = concentration =?

l = length of cuvette = 1cm

= molar absrobptivity = 5793 M -1 cm-1

0.565 = concentration X 1 X 5793

concentration = 0.565 / 5793 = 9.75 X 10^-5 M ( in the cuvette)

b) concentration in 10mL will be

M1V1 = M2V2

M1 X 1 = 9.75 X 10^-5 X 25

M1 = 2.43 X 10^-3 Molar

c) concentration = 2.43 X 10^-3 = Mass of solute / molecular weight of solute X volume of solution

2.43 X 10^-3 X 260.73 X 10 mL = Mass of solute

Mass of solute = 6.335 milligrams

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