Question

You dissolve a compound with a molecular mass of 271.98 g/mol 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 329 nm was 0.485 in a 1.000-cm cuvet. The molar absorptivity for this compound at 329 nm is ε329 = 6133 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?

Answer 1

A = Ecl,

A = absorbance     = 0.485

E = absorption coefficient or absorptivity = 6133 M-1*cm-1.

l = length of the path of light through the sample or the thickness of

the cell (cuvette) = 1 cm

c = concentration of the substance = ?

a)

concentration of the compound in the cuvet = c = A/EC

= (0.485/(6133*1))   = 7.91*10^-5 M

b) MASS of compound in 1 ml aliquot = (25/1000*(7.91*10^(-5))*271.98

    = 6.446*10^-4 grams

in 10 ml = 6.446*10^-3 grams

concentration = ((6.446*10^(-3))/271.98)*(1000/10)

   = 0.02 M

C) mass of compound were used to make the 10-mL solution

   = 6.446*10^(-3) grams