Question

An aspirin solution of unknown concentration was studied using ultraviolet spectroscopy. This solution had a percent transmittance of 37.3 when measured at 273 nm in a 1.000-cm cell. Aspirin is known to have a molar absorptivity of 741 M^-1cm^-1 at this wavelength.

a. Calculate the absorbance of this solution

b. What is the concentration of aspirin in this solution?

c. What would be the absorbance of a solution prepared by taking 10.00mL of this solution and diluting to 100.00mL in a volumetric flask, when measured in a 2.500-cm cell at 273 nm?

Answer 1

Gpver). % Transmillance = 31.3 af 2013 nm length og cell () = 1.000 em Trans mittance (T) = 37.3 = 0.373 at 2730m 100 Molar absorptioity (€) 2 741 M (m" at 273 am (a) Calculation of Absorbance (A): Absorbance and transmittance are related as: Absorbance , A = -logt L A = -109 (0.373) = +0.428 (b) Calculation of concentration of Aspirin so This can be calculated using relation: A = Ed (Beer-Lambert Law) where, A - Absorbance, E = molar absorptiority, . c = concentration and, e = length of cell ce 2 Coriem "on) I concentration, c = 5.78X10'MI (6) (as moles remains concentralen "="y Vs 10ml of this solution is diluted to 100ml For calculation of new concentration (molanity) we will use: Move = Me Vs (as moles rem Mer initial molan'ty (conc.) = 5-78x1024M Vi = initial volume = 10m! ų = final molasity (conc.) yf a final volume = 100m)

Mg = Mivi = 5.78X1024x10 5.78X105m loo un Length of cell, i = 2.5cm - Eremain sama i. e, = 4411 am Absorbance of solution, A.= sel = 941 M1 cm x 5.78 o is x 2.5cm ---D# = 0.107 Absorbance = 0.109