An unknown amount of a compound with a molecular mass of 287.27 g/mol is dissolv

Question

An unknown amount of a compound with a molecular mass of 287.27 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 325 nm is 0.518 in a 1.000-cm cuvette. The molar absorptivity for this compound at 325 nm is element_325 = 6657 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)

Absorbance=molar absorptivity×path length ×concentration

Concentration=Absorbance/(molar absorptivity × path length)

= 0.518/(6657 M^-1Cm^-1×1CM )

= 7.78×10^-5M

b) 1 ml taken from 10ml flask and diluted to 25ml

So, dilution = 25/1 = 25time

concentration in10ml flask = 25×7.78×10^-5M=1.94× 10^-3M

c) Concentration = 1.94×10^-3M

No of mol = (1.94×10^-3/1000)×10

= 1.94×10^-5

Molar mass = 287.27g/mol

Mass = 1.94×10^-5 × 287.27

= 5.57×10^-3g

=5.57mg

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