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Redefining the Mole

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Re: Redefining the Mole

#11

Post by zaarn »

sesqu wrote: post #10
WalterGR wrote: post #3

The definition of a mole provided in this article is no doubt correct, but it’s not how it was taught in my high school chemistry class. For others with the same nagging thought, the explanations of Avagadro’s Law feel more familiar: https://www.britannica.com/science/Avogadros-law In particular: The specific number of molecules in one gram-mole of a substance, defined as the molecular weight in grams, is 6.022140857…

The article does briefly discuss a slightly more accurate version of that definition, and why it's bad (it's convoluted and relies on the poorly-defined kilogram).

The kilogram will get a better definition this year if nothing holds it up (atleast, redefining it based on the planck's constant is planned for the 2018 meeting according to WP).

Re: Redefining the Mole

#12

Post by a1369209993 »

jpallen wrote: post #7
rasteau wrote: post #5

> In practical terms, the mole helps chemists measure stuff. It helps express the amounts of atoms or molecules in a chemical reaction. Cause a half-mole of oxygen molecules (O2) to react with a mole of hydrogen molecules (H2) and you get a mole of water (H2O)—equal to about 18 grams of substance. Every example I find describing the utility of the mole could just as plausibly substitute "dozen" or "googol" for "mole"…

It's an arbitrary number, but it's nice because one mole of atoms with atomic mass number X will weigh approximately X grams. This is exactly true for carbon-12 (and is what defines a mole).

> one mole of atoms with atomic mass number X will weigh approximately X grams

Speaking as someone who has had to deal with rounding errors in floating-point graphics, data structure layouts, and real estate cartography, that sounds horrifying and insane.

Re: Redefining the Mole

#13

Post by thrower123 »

bluenose69 wrote: post #8

Re the educational aspect -- when I ask students in my university class whether they remember Avogadro's number from high school, they all respond in the affirmative. When I ask them for the mantissa, the whole class sings out "6.02". Great! But, when I ask for the exponent, they are really quite uncertain. For many, the rote learning has cut off after the "times ten to the" in the sentence. This is disappointing, bu…

I remember that there is such a thing, and the relation between the number of molecules and a gram. But the exact number? No way - that's just not useful information to keep in my head post junior year chemistry. I've never had cause to need that information, especially when it is a google away.

Re: Redefining the Mole

#14

Post by JoeAltmaier »

thrower123 wrote: post #13
bluenose69 wrote: post #8

Re the educational aspect -- when I ask students in my university class whether they remember Avogadro's number from high school, they all respond in the affirmative. When I ask them for the mantissa, the whole class sings out "6.02". Great! But, when I ask for the exponent, they are really quite uncertain. For many, the rote learning has cut off after the "times ten to the" in the sentence. This is disappointing, bu…

I remember that there is such a thing, and the relation between the number of molecules and a gram. But the exact number? No way - that's just not useful information to keep in my head post junior year chemistry. I've never had cause to need that information, especially when it is a google away.

That's kind of a self-fulling prophesy or something. Not having knowledge about something means, we work around it, fake it or just ignore it.

For instance you might have wondered how much weight you lose with each breath. Converting O2 to CO2 means you're losing some mass of carbon each exhalation and that's a major channel for weight loss.

But not remembering much about chemistry, nothing comes of this. Another part of our lives remains shut off because its easier to ignore it.

Not terribly important I guess. But add up the thousands of times we move ahead without real information or curiosity, and our lives are diminished.

No, I'm not ready to be proud of how I lost most of my technical knowledge about the world, and how I blunder on in ignorance because its easier.

Re: Redefining the Mole

#16

Post by MrEfficiency »

a1369209993 wrote: post #12
jpallen wrote: post #7

Earlier quoted context omitted.

It's an arbitrary number, but it's nice because one mole of atoms with atomic mass number X will weigh approximately X grams. This is exactly true for carbon-12 (and is what defines a mole).

> one mole of atoms with atomic mass number X will weigh approximately X grams Speaking as someone who has had to deal with rounding errors in floating-point graphics, data structure layouts, and real estate cartography, that sounds horrifying and insane.

Chemistry doesnt exist in a vacuum(mostly) and usually exists as an equilibrium.

The moment you make water, some of that water has turned into CO2 and the water is a bit more acidic.

That 'some' can be 1 molecule. Would that affect anything at macro scales?

The answer is yes, yes it does. That's why chem engineers get creative in their real world applications.

Re: Redefining the Mole

#17

Post by rasteau »

cowsandmilk wrote: post #9
rasteau wrote: post #5

> In practical terms, the mole helps chemists measure stuff. It helps express the amounts of atoms or molecules in a chemical reaction. Cause a half-mole of oxygen molecules (O2) to react with a mole of hydrogen molecules (H2) and you get a mole of water (H2O)—equal to about 18 grams of substance. Every example I find describing the utility of the mole could just as plausibly substitute "dozen" or "googol" for "mole"…

The purpose of the mol is to be a convenient measure for us working in the SI unit off grams. We need to get between grams and a count of molecules, that's the number. Sure, we could work in dozens, but then we would have some other arbitrary constant we would have to memorize to go from grams to a count of molecules. And the nice thing about mols is that you don't have to memorize Avogadro's number to use them; whil…

>We need to get between grams and a count of molecules, that's the number.

I think that number is only true for carbon-12.

>but then we would have some other arbitrary constant we would have to memorize to go from grams to a count of molecules.

I think we already have to do that, hence molar mass.

Re: Redefining the Mole

#18

Post by apo »

On November 16, 2018, representatives from more than 50 countries are expected to make history when they gather in Versailles, France, to vote on redefining the SI, including the mole. The vote will close the book on this chapter of Vocke and Rabb’s work, but will open a new chapter in chemistry as the fundamental unit, for that branch of science will no longer be tied to a physical object but a constant of nature.

Interesting to find out that the concept every chemistry student is taught is about to be redefined. Odd that they couldn't schedule it on mole day...

I remember struggling with the concept of the mole in high school.

After working many problems I was able to see what the big deal was about:

The mole links the macroscopic world we can directly experience with our senses to the atomic world which we cannot.

Think of the mole as a monetary exchange rate between these two worlds. It converts mass of a pure sample (which we can measure directly on the bench top) to number of particles (which we can't). Chemistry and accounting have a lot in common. If you're good with money, you should be good at chemistry.

Anyone can pick up an ingot of silver, place it on a balance, and read the number to get the mass. Use of the mole (and the atomic weight of silver) allows this measurement to be converted into the number of silver atoms in the sample. This process is identical to the one you'd use to figure out how much your hotel in Paris will cost you in dollars.

Re: Redefining the Mole

#19

Post by pbhjpbhj »

bluenose69 wrote: post #8

Re the educational aspect -- when I ask students in my university class whether they remember Avogadro's number from high school, they all respond in the affirmative. When I ask them for the mantissa, the whole class sings out "6.02". Great! But, when I ask for the exponent, they are really quite uncertain. For many, the rote learning has cut off after the "times ten to the" in the sentence. This is disappointing, bu…

6E23! It rhymes. If they can remember "6E" and that it rhymes, then the first number after 3 they come to that rhymes is 23.

    Atoms in 1g
    Of 12C?
    It's 6E 23.

Re: Redefining the Mole

#20

Post by ChrisLomont »

a1369209993 wrote: post #12
jpallen wrote: post #7

Earlier quoted context omitted.

It's an arbitrary number, but it's nice because one mole of atoms with atomic mass number X will weigh approximately X grams. This is exactly true for carbon-12 (and is what defines a mole).

> one mole of atoms with atomic mass number X will weigh approximately X grams Speaking as someone who has had to deal with rounding errors in floating-point graphics, data structure layouts, and real estate cartography, that sounds horrifying and insane.

There is no other way. The problem is atoms combine in integer ratio amounts to form molecules. To mix things for chemistry one needs to be able to mix things in proper proportions. So having a number that trades number of atoms to something plausibly measurable, like mass, is needed.

The reason it cannot be exact for all atoms is forced on us by nature: atoms come in isotopes, each weighing slightly differently, and most common elements come in a mix of isotopes.

So picking one isotope of one element (carbon-12) as the definition for a mole that is decently representative of how chemists will use the number is a perfectly fine and useful number.

Any chemist that needs to worry about the fuzz will understand this and act accordingly. For example, carbon 22 has a mass slightly larger than 22/12 that of carbon 12 (but has short half-life). Carbon 13, which is stable, has mass slightly over 13/12 that of carbon 12, and when using it, one adjusts accordingly. And these "slightly over" phrases are also known to many digits of precision.

But nailing down the number precisely is extremely useful.

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