How to relate the transmittance to the absorbance, concentration, and molar absorptivity?
1 Answer
The absorbance
#bb(A = -log T)#
#color(blue)(A) = -log(0.762) = color(blue)(0.118)#
What would the absorbance be if the percent transmittance was
#color(white)("The substance absorbs no incoming light at all.")#
#bb(A = epsilonbc)# ,where
#epsilon# is the molar absorptivity in#"L/mol"cdot"cm"# ,#b# is the path length of the cuvette (usually#"1 cm"# ), and#c# is the concentration in#"mol/L"# . In this case,#b = "1.5 cm"# !
Therefore, the molar absorptivity is:
#color(blue)(epsilon) = A/(bc)#
#= 0.118/("1.5 cm" cdot "0.0802 M")#
#= color(blue)("0.981 L/mol"cdot"cm")#
This makes sense, because if the absorbance is low, the substance reflects most of the light that approaches it. Since absorbed light is not seen, the higher the absorbance, the darker the substance.
The darker the substance, the higher its
Let's see if that's true.
#color(blue)(c) = A/(epsilonb)#
#= 0.118/("10000 L/mol"cdot"cm"cdot"1.5 cm")#
#= color(blue)(7.87 xx 10^(-6) "M")#
Indeed, the concentration is very low.