Can "mole" unit only be integer values?

[pyroknife]

I am confused about the concept of a mole. Avogadro's # says that 1 mole has 6.023*10^23 particles.
Does it make sense to talk about moles in a non-integer sense?
For example, can you say that 1/2 mole has 1/2*(6.023*10^23) particles?

 

[phyzguy]

I am confused about the concept of a mole. Avogadro's # says that 1 mole has 6.023*10^23 particles.
Does it make sense to talk about moles in a non-integer sense?
For example, can you say that 1/2 mole has 1/2*(6.023*10^23) particles?

Yes, you can certainly talk about 1/2 mole, which has 3.0115*10^23 particles, or 1/10 mole, which has 6.023*10^22 particles, or 1/1000 mole, which has 6.023*10^20 particles. Perhaps your confusion lies in the fact that you must have an integer number of particles. But Avogadro's number is so huge, that you cannot resolve it down to the level of individual atoms. To do this, you would have to tell the difference between the number:
602,300,000,000,000,000,000,000
and the number:
602,300,000,000,000,000,000,001
No measuring device is that precise. So you are always talking about an approximate number of particles.

 

[Borek]

Actually there is a proposal to redefine mole to be an integer number.

Technically it IS an integer number even now, as it is "number of atoms in exactly 12 g of C-12". We are just limited by the accuracy with which we can define and measure mass.

 

[SteamKing]

I am confused about the concept of a mole. Avogadro's # says that 1 mole has 6.023*10^23 particles.
Does it make sense to talk about moles in a non-integer sense?
For example, can you say that 1/2 mole has 1/2*(6.023*10^23) particles?

Now, if you'd like to volunteer to count the number of atoms in 12 g of carbon ...

 

[fresh_42]

Didn't they try to redefine 1kg as a certain and exact amount of atoms?

 

[SteamKing]

Didn't they try to redefine 1kg as a certain and exact amount of atoms?

That's just one proposed method for defining the kilogram:

https://en.wikipedia.org/wiki/Kilogram

See the section on "Atom Counting"

There are other proposals for defining the kilogram in terms of certain fundamental constants, like Planck's Constant, but no agreement has yet been reached by the international body in charge.

 

[fresh_42]

Thank you. I thought they were closer.

 

[James Pelezo]

Now, if you'd like to volunteer to count the number of atoms in 12 g of carbon ...

Well, by NIST standards N_a = (6.0221415 ± 0.0000010) × 10²³ units, provided that atomic mass of C-12 remains constant over time... but, of course it does not ... Interesting challenge.

 

[DrClaude]

provided that atomic mass of C-12 remains constant over time... but, of course it does not ...

What do you mean by that? If you are talking about the fact that the mass of the International Prototype Kilogram in Paris changes with time, this has nothing to do with the properties of ¹²C changing, but is simply related to the imperfections of an actual artifact.

 

[DrDu]

Even the number of particles can be non-integer, e.g. when you talk about statistics, so there is even less problem with the number of moles being non-integer.

 

[James Pelezo]

What do you mean by that? If you are talking about the fact that the mass of the International Prototype Kilogram in Paris changes with time, this has nothing to do with the properties of ¹²C changing, but is simply related to the imperfections of an actual artifact.

In this case, I can only reply to what I have read. The note specifically postulates that C-12 has changed with time, but, granted, it does not specify anything relating to properties of C-12 ... Here's the excerpt and reference...

These changes cannot be measured exactly, simply because there is no "perfect" reference against which to measure them—Le Gran K is always exactly one kilogram, by definition. It is estimated that Le Gran K may have changed about 50 micrograms—that is, roughly by about 150 quadrillion (1.5 × 1017) atoms—since it was constructed. This implies that by current measurement conventions, the mass of a single atom of carbon-12 is changing in time, whereas modern theory postulates that it remain constant.
http://www.americanscientist.org/issues/id.368,y.0,no.,content.true,page.1,css.print/issue.aspx

My apologies if I've misunderstood your point of issue.

 

[DrClaude]

The key part of the quote is

This implies that by current measurement conventions, the mass of a single atom of carbon-12 is changing in time,

The mass is apparently changing because the standard keeps changing as the IPK loses or gain some atoms as it is manipulated.

 

[Dale]

This implies that by current measurement conventions, the mass of a single atom of carbon-12 is changing in time, whereas modern theory postulates that it remain constant.

This is poorly worded. The mass of an atom is not changing over time. The mass of the kilogram prototype is changing over time. So as the mass of the kilogram changes the number of atoms per kilograms must change in order for the mass to remain constant.

 

[James Pelezo]

This is poorly worded. The mass of an atom is not changing over time. The mass of the kilogram prototype is changing over time. So as the mass of the kilogram changes the number of atoms per kilograms must change in order for the mass to remain constant.

I'm confused by the wording of the underlined statement... 'as the mass of the kilogram changes the number of atoms per Kg must change => constant mass'? This sounds like a 'Yogi-ism'... 'If you come to a fork in the road, take it' ... confusing, huh? That is, how does the number of atoms changing result in a constant mass?

 

[DrClaude]

I'm confused by the wording of the underlined statement... 'as the mass of the kilogram changes the number of atoms per Kg must change => constant mass'?

He means "as the mass of the kilogram prototype changes".

 

[James Pelezo]

OK, thanks... that makes more sense.