Beer's Law

[dsl]

A solution containing 3.5 mmoles/l of an absorbing compound was diluted 1 in 80 and then read at 670nm in a spectrophotometer using a 1cm sample holder.
If the measured aborbance was 0.37, what was the corresponding transmittance?Calculate the molor absoptivity of the compound at the wavelength used.

[joggerjayne]

42

[factor-fiction]

Is 42 the transmittance or the absoptivity (has dsl made this word up?), or is it both, Joggerjayne.

In my day we had to do our own homework dsl

[eltelioni]

factor30 - I don't want to be pedantic but the absence of units indicates that 42 is the transmittance.
If it were the molor absoptivity I'm sure joggerjayne would have said 42 M^-1 cm^-1.
That still leaves the question regarding the absoptivity. Do you want dsl to fail?

[joggerjayne]

That's what I was going to say.

[eltelioni]

Oops, sorry joggerjayne. I thought this was a wind-up, you know, 42 being the answer to the great question of life, the universe and everything. But, darn me, the answer is 42!

[dsl]

thanks guys for the help!! and your concerns bout not wanting me to fail 'eltelioni'!! but 'factor 30' its not that i just want the answer for homework i really want to understand how u guys were able to work it out so i know for future reference. i just cant seem to figure it out how you guys mastered the answer 42???

[eltelioni]

This could be tricky in plain text, but here goes.
A = absorbance
T = transmittance
I = intensity of light after passing through the sample
J = intensity of the light before passing through the sample
A = -log(I/J) using log to base 10
and T = I/J
so A= -log(T)
so T = 10^(-A) (10 raised to the power -A)
so T = 10^(-0.37)
using Google as a calculator, that gives 0.426579519
which I would round to 0.43
Transmittance is often expressed as a percentage, so multiply by a hundred to give 43.

[dsl]

wow....i actually get it now. i was so close, but kept doing sumthing small wrong. thank you so much!

[joggerjayne]

Aah. I rounded 0.426579519 the wrong way and took it down to 0.42.

eltelioni is right ... it shoud be taken up to 0.43

[joggerjayne]

l

Here, I just found the "l" that was missing from that last post.