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# The mole and Avogadro constant...

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1. (Original post by shohaib712)
I think I get it:
If a question asked about the mass of an atom - we can use the Ar/Mass number?
but if it asks about a specific Isotope or ion - we have to calculate the mass individually but adding up the Relative masses of its subatomic paticles?
Yeah if it doesn't specify the isotope just use the relative atomic mass, as any sample you have in reality will be a mixture of the isotopes.

Do what I said in my post above this for working out isotopic masses. The amu is a quantity which is there for a reason, but OP wasn't given it to work out their question so they have to add the individual nucleon masses.
2. (Original post by alow)
The mass number on a periodic table is the relative atomic mass.

Working out the actual mass of an isotope is quite difficult due to binding energy. Just add the mass of the constituent particles to get a decent approximation. Sometimes this is better than using 1amu for both, sometimes it isn't. Depends on the nucleus.

Neutrons are slightly heavier than protons. The amu is somewhere between them.

What I would do for 1H, is just add the mass of a proton and an electron.

For 1H+ just use proton mass.

For 1mol of 3H, use 3*amu + 1*electron mass, then multiply it by Avogadro's number.
Why do we define the mass number as the number of neutrons and protons in an atom, if it hapens to be the average mass of all isotopes abundances in an element?

Oh I see - Do you know any resources that explains binding energy? - does it only affect isotopes' masses?
3. (Original post by alow)
Yeah if it doesn't specify the isotope just use the relative atomic mass, as any sample you have in reality will be a mixture of the isotopes.

Do what I said in my post above this for working out isotopic masses. The amu is a quantity which is there for a reason, but OP wasn't given it to work out their question so they have to add the individual nucleon masses.
Alright!
And the amu - is bassically the unit for the masses ofatoms (1/12th of mass of c-12 or 1amu is equivalent to 1 proton/neutron?
4. (Original post by shohaib712)
Why do we define the mass number as the number of neutrons and protons in an atom, if it hapens to be the average mass of all isotopes abundances in an element?

Oh I see - Do you know any resources that explains binding energy? - does it only affect isotopes' masses?
The mass number is the number of neutrons and protons in a nucleus. The number on a periodic table is the Ar. The mass number is usually not a 100% accurate representation of the mass of the atom though.

This is alright: http://hyperphysics.phy-astr.gsu.edu...ne/nucbin.html

Calculating the binding energy is difficult, working it out from experiment is relatively easy. Binding energy effects any nucleus that isn't protium's mass.
5. (Original post by shohaib712)
Alright!
And the amu - is bassically the unit for the masses ofatoms (1/12th of mass of c-12 or 1amu is equivalent to 1 proton/neutron?
Yeah the amu is used because it considers the binding energy of Carbon-12, which has a typical binding energy per nucleon.
6. (Original post by alow)
Yeah the amu is used because it considers the binding energy of Carbon-12, which has a typical binding energy per nucleon.
Thanks for the help! I'll check the link out!
7. (Original post by alow)
For 1mol of 3H, use 3*amu + 1*electron mass, then multiply it by Avogadro's number.
Sorry to intrude, but I'm working on some similar exercises and have a question. For the first two, I can understand what to do but for the third part, why do we multiply it by Avogrado's number to find the mass of one mole?

I feel like something just flew above my head and I'm just not grasping onto it.
8. (Original post by bmgaic)
Sorry to intrude, but I'm working on some similar exercises and have a question. For the first two, I can understand what to do but for the third part, why do we multiply it by Avogrado's number to find the mass of one mole?

I feel like something just flew above my head and I'm just not grasping onto it.
The first two ask for the mass of an individual isotope/ion. The third asks for the mass of a mole of an isotope.
9. (Original post by alow)
The first two ask for the mass of an individual isotope/ion. The third asks for the mass of a mole of an isotope.
Okay, I've got it. Thank you!

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