Elements and Moles

In this General Chemistry lecture, we discuss the structure of the Periodic Table of Elements. We also introduce Avagadro's number and the concept of the mole. A sample problem is worked using moles and molar masses.

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Education

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• chemistry
• General Chemistry
• Wight
• Utah
• Periodic Table
• Avagadro
• mole

[suddenlyitsobvious]

(4) -presumbaly beacuse it's (as always) really all about MODELS and abstractions?

[chemdog8]

@suddenlyitsobvious The concept of the mole has tseveral functions, any of which can be accomplished without the mole concept, but nevertheless exists for CONVENIENCE of use:

1. Just as a dozen is shorthand for 12, a mole is shorthand for 6.02E23 (the number of C-12 atoms in 12 grams).

2. Although the mass ratio of H to Cl in HCl is 0.02344:1, the mole ratio is 1:1, which is more intuitive.

3. While the mass of a single O2 molecule is 5.3138E-23 g, the mass of 1 mole of O2 is 32 (much easier).

[suddenlyitsobvious]

(5)

I wonder if there's any PROOF that same volumes of gas at identical pressures

have equal amounts of particles? Or is this indeed an ASSUMPTION?!

[chemdog8]

@suddenlyitsobvious The proof that equal volumes of gas at the same pressure contain equal numbers of molecules comes from the reactions themselves. Equal volumes of hydrogen and chlorine will react to form the same total volume of HCl with nothing left over. This builds on the idea that a pure substance consists of identical molecules that contain small integer numbers of atoms of specific types. But the logic is progressive, not circular, and the proof is rigorous.

[hiaaa2]

how many moles are in 8.99*10^10 atoms of argon

[chemdog8]

@hiaaa2 Jimmie the Geek says 1.5E-12, but I say, "None, silly, argon is a gas."

[suddenlyitsobvious]

@chemdog8

(4)

...to translate it to actual weights in grams.

So my issue with this is that the mole, that I quite understand to be CONVENIENT

for chemists, appears to be a hypothetical entity based on ASSUMPTION, generating the

illusion that atomic weights and the number of atoms in a certain volume

at fixed pressure is established, when this is really not the case.

[suddenlyitsobvious]

(3)

So equal volumes of different gases at equal pressures are assumed

to have equal numbers of particles -atoms- wherefore the difference in

weight between those volumes is taken to be the difference in weight between

the different individual atoms.

Because the system of atomic weights only gave the relative weights of

atoms and molecules, chemists needing some kind of 'golden reference' they

could work with came up with the "gram molecular weight" or "mole",...

[suddenlyitsobvious]

(2)

...as another filled with oxygen, both recipients will contain

the same number of particles, even though the one filled with oxygen

will be much heavier.

This principle allowed to compare the weights of the particles

in the gases and since two volumes of hydrogen combined

with one volume of oxygen to produce water, it was established

that there were two hydrogen atoms for each oxygen

atom.

[suddenlyitsobvious]

@chemdog8

Thank you for your response. I've looked into this some more and can

now formulate my query somewhat more precisely;

Avrogadro (1776:1856) provided an explanation for Gay-Lussac's 'law of

combining ratios' (volumes of gases reacting with one another did so

in ratios of small integers), suggesting that at a fixed temperature and

pressure, equal volumes of gas contained the same number of particles.

Meaning if a recipient is filled with hydrogen to the same pressure...

[suddenlyitsobvious]

(3)

1.00794 / (15.9994/2) = 0,12604

0.12604 * 5.5 grams = 0.6932 grams of H2

Where's the need for the mole?

Does the mole serve to create a system of floating references circumventing the need to admit that it simply cannot be established how much an atom ACTUALLY 'weighs' (sorry) in terms of grams & that this ploy couples relative weight to VOLUMES of atoms because attempts to determine actual weight of an individual atom have simply failed...

[suddenlyitsobvious]

(2)

Could it be that science simply has not been able to formulate -and I'm using figurative layman's terms here- what 'the scales' would indicate were it possible to simply put an atom of Oxygen on them, or H, or Fe? Anyway, if the mass (whatever that means in this interdepent system of references -does anybody even know?) of H is 1.00794, and of oxygen (O2) 15.9994 then the calculus to arrive at the same result can simply be: