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Topic: calculate mass one mole Ag atoms?  (Read 5138 times)

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Offline Phlegethon Borsalino

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calculate mass one mole Ag atoms?
« on: July 06, 2015, 01:16:40 PM »
I am not a student trying to get answers to my homework. I am taking a chem course for personal knowledge.

There is probably a tutorial on this somewhere, so perhaps someone can redirect me.

I want to calculate the mass in grams of one mole Ag (or any element).

I know that we just take the amu and tack on grams to that, and that is the weight of a mole of those atoms, but I want to know why.  I know it can be done without a calculator.

when I try it seems to require a calculator.

I know 1g = 6.022x1023 amu

and I know 1 amu = 1.6 x 10-24g

I have tried to convert these into conversion units like
1g/6.022x1023

and 1amu/1.6x10-24

But I get bogged down.

I already know that one mole Ag = 108 g. (Because the amu of Ag is 108   But I would like to know how to calculate this.

Offline Borek

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Re: calculate mass one mole Ag atoms?
« Reply #1 on: July 06, 2015, 03:41:00 PM »
1 mole is how many atoms?

If an atom mass is 108 amu, what is mass of that many atoms?
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Offline Phlegethon Borsalino

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Re: calculate mass one mole Ag atoms?
« Reply #2 on: July 09, 2015, 04:12:10 PM »
1 mole is how many atoms?

1 mol of single atoms= 6.022 x 1023 single atoms.

If an atom mass is 108 amu, what is mass of that many atoms?

So you seem to be saying that I should just multiply 108amu x (6.022x1023

This actually means 108 x mass of one amu x 1 mol.

1 amu = 1.6 x 10-24

That would require a calculator.  I think there is a simpler way. I am just missing it.

Ok, if you mean one mole of Ag. That would be 108 amu. So 108.... no, I'm lost here.

Offline Borek

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Re: calculate mass one mole Ag atoms?
« Reply #3 on: July 09, 2015, 05:38:39 PM »
The simpler way is the one you have already mentioned - take mass expressed in amu, and switch to grams. It works by design, and doesn't require a calculator. However, to find out what the amu value (amu mass expressed in grams) should be for the idea to work, you need to use a calculator.

So you seem to be saying that I should just multiply 108amu x (6.022x1023

This actually means 108 x mass of one amu x 1 mol.

Try with a calculator:

6.022×1023 atom/mol × 108 amu/atom × 1.66×10-24 g/amu

Check both what is the number you got, and what is the unit you got.
« Last Edit: July 12, 2015, 02:49:18 AM by Borek »
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Offline Enthalpy

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Re: calculate mass one mole Ag atoms?
« Reply #4 on: July 12, 2015, 06:37:29 AM »
The question is a bit misleading because natural Ag is a mix of 107Ag and 109Ag whose mean value is ~107.87, so 108 shouldn't be understood as an integer number.
http://www.webelements.com/silver/isotopes.html

Because the neutron happens to weigh about as much as a proton plus an electron (this lead to wrong theories in the past), and because a neutral atom has as many electrons as protons, one can multiply the number of nucleons (107, 109...) by the mass of one neutron to approximate the mass of the neutral atom.

Though, elements show deviations to that, which are commonly observed. Beyond neutrons' mass differing from protons plus electrons, this is because energy has a mass, and the interaction between the nucleons is so strong that the mass difference is seen on lab scales.
https://en.wikipedia.org/wiki/Nuclear_binding_energy
So even for monoisotopic matter, the number of amu isn't an integer. Compare 4He (4.0026) with 59Co (54.942).

Because these deviations have practical implications, some convention is needed for the amu. Carbon is in the midrange between a small (He) and a big (Co) mass defect, and many atoms in organic chemistry have a mass defect similar to carbon: useful. On the other hand, natural carbon comprises 1% 13C too, so the standard specifies 12C.

Offline Borek

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Re: calculate mass one mole Ag atoms?
« Reply #5 on: July 12, 2015, 06:50:22 AM »
What you posted is just confusing and not related to the original question. Problem here is with the very basic relationship between a mole, mass of an atom, and mass of a mole.

You don't need binding energy nor general relativity, nor string theory to answer that.
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Offline Enthalpy

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Re: calculate mass one mole Ag atoms?
« Reply #6 on: July 12, 2015, 09:30:58 AM »
So that's why my post had silently disappeared.

To the very least, it's useful to tell that no natural silver atom has 108 nucleons.

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