Some questions concerning Chemistry

[Resign the King]

I was wondering, when chemists created the Atomic Mass Unit, and the Mole how did they determine the number of atoms?

I understand that the Atomic Mass Unit is chosen randomly, they use the mass of one element, and the ratio of the two elements of a compound to show how much more one weighs than another.

An example from my text: "x=the number of atoms in the element. A compound contains equal numbers of Ni and O atoms. The sample contains 8.15g of Ni and 2.22g of O. (8.15g)/x / (2.22g)/x = 3.67. The atomic mass of oxygen has been established at 16.0 amu. So 3.67 (16.0 amu) = 58.7 amu of Ni."

I understand that they choose the amu of one element randomly and then from there they can determine the masses of other elements compared to it. So taking water, they could see that the ratio of the masses of H and O when separated would be 8:1. But how is it that they determined that there were 2 atoms of H for every O making the ratio 16:1?

So how did chemists know, when determining the amu of other elements based on the amu of a randomly chosen element, the number of atoms of each element in the compound?

Also a mole is defined as the number of 12C atoms in 12g of 12C. How do chemists know this?

[Mantikore]

the funny thing is, at the moment i was reading this, i was reminded that today is mole day!
http://en.wikipedia.org/wiki/Mole_Day

anyway, im not sure about why carbon was used in preference to oxygen. i would have thought carbon has more isotopes to mess up the measurement

[JoePedo]

Some questions concerning Chemistry

:mad: did you perhaps consider the chemistry forum??? :mad:


I was wondering, when chemists created the Atomic Mass Unit, and the Mole how did they determine the number of atoms?

By making shit up.

Honestly, though, I don't have the EXACT answer, just a bit of random tidbits of history... first, let's fix your question...

I was wondering, when chemists created... the Mole how did they determine the number of atoms?

I have no idea if this pertains, but around the time of the US revolutionary war, some guy calculated the size of a molecule of oil.

He did this by chucking a millilitre or so in a lake, and measuring the surface area of the slick. Once you've got two dimensions worked out, you can calculate the thickness of the slick - presumably monomolecular - for the same total volume, and, voila, that's how big the molecule is.

From there, it's not too hard to count them. 'n that started some interesting shit, I'm sure... however...

I understand that the Atomic Mass Unit is chosen randomly

No, no it's not.

It's the weight of one baryon. Give or take a lepton, but since the weight of a lepton is less than the mass supremacy of the neutron over the proton (remind me to use the phrase "mass supremacy of the neutron over the proton" more often), that's way, waaayyy beyond their accuracy at the time.

Since everything is made up of protons and neutrons (and leptons), it wasn't so much "random," as "the weight of one building block" - 1H1.

Incidentally, I think we get increased federal attention around here every time we write hydrogen isotopes in the nHn format. ;) 1H1 probably isn't even scanned for, however...

So taking water, they could see that the ratio of the masses of H and O when separated would be 8:1. But how is it that they determined that there were 2 atoms of H for every O making the ratio 16:1?

PRO TIP : 1 mol of any gas occupies 22.4 L volume at standard temperature and pressure...

...or more specifically... any idiot could do a little electrolysis and notice a 2:1 volumetric ratio. ;)

So how did chemists know

Maybe you should have asked the chemists?

Also a mole is defined as the number of 12C atoms in 12g of 12C.

Ehwut?

Ah well...

How do chemists know this?

Well, see the oil-measuring tidbits above. Probably a few other experiments along those lines, too.

More specifically, though, a mole is defined as the number of atoms in (atomic weight) grams of (element). Since atomic weight is, well, determined - you can pick up averaged numbers for naturally-occuring isotope spreads and everything, hydrogen is a little over 1 even though 1H1 is definitionally an atomic weight of 1, until our understanding of atomic theory changes drastically...

The rest of the story is, well, insanely varied... but if you set fire to something, you can trap the water and CO2 and H2NO3 and such and weigh them. Ratios will start appearing, compare them to one another, notice some specific trends and quanta. Certain chemical reactions/tests start defining organic groups, and these regularly append similar data. Pretty soon, people start learning that methane will always provide 2 H2O to 1 CO2. Yeah, those equilibrium formulas? Some old shit, homie, probably dating way back before we knew most of what we know and the first chemistry beyond raw experimentation and distillation.

It doesn't take long before that drop of oil on the surface is a drop with known properties. Nor does it take long to figure out that most elements seem to weigh in multiples of hydrogen. 6.22 * 10^23 is a wierd-ass number, and a giant freaking road sign that someone blurted out "umm... a GRAM of hydrogen??" long before people started counting atoms, back when they were weighting fractions of degredates and counting quanta against each other...

It's simple hacking. One mol of butane will produce 5 mol H2O and 4 mol CO2 with unlimited oxygen. 5 mol H2O will produce 5 mol of H2 and 2.5 mol O2... but it will produce 10 mol HCl and 2.5 mol O2... trippy, huh?

'n CO2 weighs about one oxygen more than carbon monoxide. You do the math. ;) After all, somebody else did - and it's all hacking out quanta and atomos, bit by bit... taking the last, best guess at the indivisible, and waiting for someone to break it with a new experiment.

Then they got to subatomic particles, and they threw up their hands and stopped calling atom-hacking chemistry... but that's a different story.

Post in the right forum next time, k?

[harry_hardcore_hoedown]

the funny thing is, at the moment i was reading this, i was reminded that today is mole day!
http://en.wikipedia.org/wiki/Mole_Day

anyway, im not sure about why carbon was used in preference to oxygen. i would have thought carbon has more isotopes to mess up the measurement

Goddamnit! I always learn this a day too late. Also, I think carbon is used because diamond is very pure.

[IVI]

http://www.lmgtfy.com/?q=definition+the+unified+atomic+mass+unit