Well, the units of "molar" are cm^-3 ("dimensionless number per volume"). So your molar extinction can also be expressed in cm² [per dimensionless number]. Then you can see that when working in mass concentration you'll need cm² per mass, which is where the cm².mg^-1 comes from. Conversion between the two requires molar mass, which has units of mass per dimensionless number.

Edit: And, leading on from what Nicholas has just said, the relationship between units, and the nature of extinction, are made clearer if the units are not cancelled down:

Molar extinction: [cm^-3]^-1.cm^-1

Mass extinction: [mg.cm^-3]^-1.cm^-1

(That is, extinction is measured per concentration, per path-length.)

Grant Hutchison

Which weighs more? A pound of feathers or a pound of gold?

And no, they are not the same.

They are if you weigh them in vacuum.
(Which is a hint, if clop is looking for the answer I think he's after. )

Grant Hutchison

I have seen the TV program on this kid he is HUGE and what is
known in the UK as a "Lardarse", I doubt if he will see 30...


David

Aren't they also the same if you weigh them in air at rest, or are you dealing with a buoyancy force of the air?

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To the regular visitor of internet bulletin boards it is clear that it's an excellent idea your parents get to choose your real name.

Sorry wrong answer. They don't weigh the same even in a vacuum. Try again.

I don't think that's the gift horse you think it is, if clop is looking for the answer I think he is.

__________________
"I have a cunning plan that cannot fail."
S. Baldrick

maybe it depends on the market not the wind.

The American forestry profession is a funky mix of systems sometimes. For most "practical" work "in the field", we normally use old traditional units, including a few that are almost forgotten outside of forestry, like the "chain" (a length/distance unit equal to 66 feet). But there's also a scientific side to the profession, and science (including forest science) uses metric units even in the USA, so the metric units have affected how we do other things now, too. For example, when measuring trees' heights and trunk diameters, we divide a foot into what I jokingly call "metric inches", which are a tenth of a foot. I think that's because it's just a bit easier to write down than real inches, with one small punctuation mark and only a single digit to deal with for the fractions. Stranger yet, I once worked on a research project where we were to identify, measure, and map every tree that was at least 3.3 feet tall. Why 3.3? Because it's as close as you can get in feet (and metric inches) to one meter, which is the real cutoff they were apparently interested in. So why not just have us measure in metric units?

Ah, so there's a sort of Trojan horse embedded in your question, sneaking in two different sets of units without labelling them as such.

Edit: After writing which, I think I just got swansont's pun

Grant Hutchison

Why I do these things is beyond me (probably some mis-guided curiosity), but...

Gold is trading at 343.123 GBP per troy ounce

3 oz of neon turkey feathers is $4.95

Applying all the necessary conversions we get
0.09 grams per GBP for gold.
and
33.73 grams per GBP for feathers.

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Numbers are not case sensitive. (me)

The possibly more confounding thing is that an ounce of feathers also has a different weight than an ounce of gold, but the inequality switches direction.

__________________
"I have a cunning plan that cannot fail."
S. Baldrick

Well, stones are all well and good for measuring the weight of a strong, stonelike body. But mines squishey. Kinda like a jelly donut. And jelly is commonly made from grapes. So I propose a new system for describing the weight of a squishy person: the grape!

I couldn't find the weight of an individual grape, but found the average weight for a "cluster of grapes" is .36lbs. We'll just call the cluster a "Grape". Hey, this is measurement, it doesn't have to make sense.

So, with this new measuring system, I weigh about 513.9 Grapes!(Gps) Wonder what they will say at the BMV when I get my license renewed and I drop that little nugget of information on them...

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I'm like one of those idiot savants...well, except for the savant part.

Theory of Zombie Relativity:
1) Everyone Else is a Zombie relative to You
2) Whether or not it matters is related to the inverse square of the distance between their teeth and your brain
(Quoted from Demigrog)

I don't use just any stones to measure my body. As my body is a cathedral, marble would be the absolute minimum.

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To the regular visitor of internet bulletin boards it is clear that it's an excellent idea your parents get to choose your real name.

Obviously you don't use stones in the US because you are a new country and haven't yet had a chance to standardise the mass of your stones, but England has been in business for a lot longer...

My wife recently bought a new set of bathroom scales, made in China, naturally, and calibrated in stones. Closer examination revealed that the small marks in between the main ones for stones divided the intervals into ten parts, not fourteen.

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"The truth may be out there, but lies are inside your head" Terry Pratchett

metric stones, now we're really doomed.

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To the regular visitor of internet bulletin boards it is clear that it's an excellent idea your parents get to choose your real name.

A cluster of grapes makes about one glass of brandy, so I would say that your units are "snifters" and the smaller divisions would be "grapes". Gemstones will be wieghed in "pits", and it can extend into the liquid measure of "squish".

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Numbers are not case sensitive. (me)

So, I should say I was 513.9 ClusterGrapes, or (CGs). Althought I just had to run out in this pouring rain, so you can probably add a few clusters to that, at least until my clothes dry.

__________________
I'm like one of those idiot savants...well, except for the savant part.

Theory of Zombie Relativity:
1) Everyone Else is a Zombie relative to You
2) Whether or not it matters is related to the inverse square of the distance between their teeth and your brain
(Quoted from Demigrog)

The best thing about Google calculator is that it understands smoots!

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Everything I need to know I learned through Googling.

I like to measure sickness in units of dogs, and (lack of) sanity in hatters. People look at me funny anyway, so what the heck.

__________________
"I have a cunning plan that cannot fail."
S. Baldrick