You dissolve a compound with a molecular mass of 271.98 g/mol in a 10-mL volumet
ID: 986417 • Letter: Y
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 329 = 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?
Explanation / Answer
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
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