A sample in a #1cm# cuvette gives an absorbance reading of #0.558#. If the absorptivity for this sample is #15000 L/(mol.cm)#, what is the molar concentration?
1 Answer
The concentrations are (a)
Explanation:
A. Molar concentration from absorbance
To solve this problem, we use Beer's Law:
#color(blue)(bar(ul(|color(white)(a/a) A = epsilonlc color(white)(a/a)|)))" "#
where
We can rearrange this equation to get
#c = A/(epsilonl)#
In this problem,
Then,
Molar concentration from transmittance
Recall that absorbance is defined as
#color(blue)(bar(ul(|color(white)(a/a)A = log(I_0/I)color(white)(a/a)|)))" "#
where
Transmittance
#color(blue)(bar(ul(|color(white)(a/a)T = I/I_0color(white)(a/a)|)))" "#
∴
Also, percent transmittance
so
∴
#color(blue)(bar(ul(|color(white)(a/a)A = 2-log(%T)color(white)(a/a)|)))" "#
This last equation is worth remembering because it gives you an easy way to calculate absorbance from percent transmittance.
Now, we can use Beer's Law: