Can anyone help with A=2-log %T ??

11/13/2010 | Me

[paul544]

Was wondering if there are any chemistry whizzes here who can help me with a problem in a lab I am working on?

We were given a list of %T values and told to use A=2-log %T to get the absorbency (A). I'm struggling with this. Can someone take me step by step through one? Say 25%?

[JRandomFreeper]

Not many arguments about .308 vs 30-06 either, over on DU.

/johnny

[CharlesWayneCT]

Actually, A can be any number from 0 to infinity.

T can be a percent from 0 to 100. 0% is infinite absorbtion — Log10(0) is undefined, and the lim x->0 of log10(X) is negative infinity.

For example, log10(0.1) is -1, so if %T is 0.1%, A is 3 (2 - - 1).

1% transmittal gives you 2, because log10(1)=0.

[Viking2002]

I want you to get that hidden camera out of my ceiling fan RIGHT NOW, buster!

"But know this, that in the last days perilous times will come: for men will be lovers of themselves, lovers of money, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, unloving, unforgiving, slanderers, without self-control, brutal, despisers of good, traitors, headstrong, haughty, lovers of pleasure rather than lovers of God, having a form of godliness but denying its power. And from such people turn away, for his name is Obama."

[devere]

Thanks for the math lesson. I’ve been away from the lab for 40 years.

[ThomasThomas]

I have a theory based on this.

The simplicity of an answer is inverse portion to the amount of hear people you ask.

[MHGinTN]

No, silly, I think he’s soaking in it ... but it’s mild!

[DBrow]

It’s usually A= log(1/T)

[Vendome]

Nope and stop doing that.

It’s funny but it’s weird. LOL

[Viking2002]

It wasn't funny and it wasn't weird. My shorts were riding up and I had to scratch. LOL

"But know this, that in the last days perilous times will come: for men will be lovers of themselves, lovers of money, boasters, proud, blasphemers, disobedient to parents, unthankful, unholy, unloving, unforgiving, slanderers, without self-control, brutal, despisers of good, traitors, headstrong, haughty, lovers of pleasure rather than lovers of God, having a form of godliness but denying its power. And from such people turn away, for his name is Obama."

[Hoodat]

It looks like you are trying to do a Beer-Lambert equation for absorbancy. The spectrophotometer you used gave you the transmission percentage of light passing through your solution medium. No absorbance of light results in T-value of 100%. Complete absorbance of light results in T-value of 0%. You need to convert this into a value for absorbance - A. By your equation, A = 2 - log(%T)

If T = 25%, then A = 0.60206. You will need this value for the next equation:

A = εlc

A = absorbance
ε = molar absorbtivity [L/(mol.cm)]
l = length of test light path through medium [cm], (usually 1 cm)
c = molarity of solution [mol/L]

In a typical Beer's Law experiment, you are searching for the concentration of an unknown sample. You do this by calibrating the molar absorptivity through a series of experiments with known concentrations. You can plot these values of concentration v. absorbance and they should be linear. The slope will give you εl.

ε = A/lc

Once the absorptivity constant is calculated, you repeat the experiment for the unknown sample to determine it's molarity. Capisce?

[cornfedcowboy]

Aren’t many arguments that can’t be settled with a .308 or 30-06 either.

[freekitty]

I made that mistake once myself. I never did it again.

[Hoodat]

A can never be negative. Basically,
A = log(100/transmittance percentage)

This can be rewritten as

A = log(100) - log (transmittance percentage)
or
A = 2 - log(%T). Since %T can never exceed 100, A will never be negative. And as %T approaches zero, A approaches infinity.

[Cacique]

Had a girlfriend that did that, she also added powdered Ajax. Was trying to clean her bathtub with the stuff. Lucky me I walked in in time. In hindsight, I should have left and made the apartment airtight. That was like 20 years ago. :-)

[JRandomFreeper]

I prefer 7,62x54. But that's me. ;)

/johnny

[Vendome]

LOL

[Vendome]

Neither did I.

It was worse than the pepper spray I allowed the Sheriff Department to spray with me as part of a training class, though that did last longer, it didn’t hurt as much as my little chemistry experiment.

Cough!!!!!, Cough!!!!!, Cough!!!!!

[paul544]

Sì. Grazie.

[Robert A Cook PE]

That's where we all start. If we stay there, that's up to us.

Me? I'm just a cook. Want bearnaise sauce on your poached egg on muffin with Canadian bacon?

Yes.

Please fax me three of your poached eggs. You may leave the bear sauce in Canada. I recommend sending sausages rather than Canadian bacon, since they will go through the wires faster.

[CodeToad]

“why don’t you answer his question and not be a smart ass?”

Because, unlike you, this isn’t domyhomework.com and we are not co-dependent and want to be his hero.