Question

Assuming a molar absorptivity of 50,000 (Liters/mol cm) calculate what molarity you would need to get an absorbance of 1.0, using Beers Law.

A =ε c l, where A is the absorbance; ε is the molar absorptivity in L/mol cm; c is the concentration in mol/liter; and l is the path length in cm, which, in our case, is 1.00 cm.

Algebraically rearranging gives c = A/ε l =A/ε.

B) Start with 20 mg of your dye. Calculate what morality would result when this 20 mg of dye is dissolved in 50 ml (0.050 liters) of acetone.

C) Then calculate what volume of this solution would be required to dissolve in 50 ml of solvent to give a solution having an absorbance of 1.0, calculated in part A.

Remember, C1V1=C2V2, or V1 =(C2V2)/C1

Answer 1

C=(1)/(50000)= 2*10^-5 mol/L

let's assume the dye is the one described below, because it is necessary to know the molar mass to proceed:

FD&C Red 40

Allura Red AC

496.42 molar mass (g/mol)

So the molarity of the solution proposed in part B would b

C=(0.020g/496.42g/mol)/0.05L= 8.06*10^-4 mol/L

Part C)

Using the Beers law, to get an absorbance equal to 1.0 is necessary a concentration of 2*10^-5mol/L as it was calculate previously. So in order to have this concentration starting from the previous solution it is necessary to use the following formula

C1*V1=C2*V2---->V1=(C2¨*V2)/C1

V1=(2*10^-5mol/L*0.05L)/ 8.06*10^-4 mol/L=0.0012L are needed in part C.