# Moles and atomic weights

1. Jul 3, 2007

### Mr_Bojingles

I can't understand how both the atomic weight of one atom of an element and the weight in grams of a mole of that element are the exact same thing.

For example helium has an atomic mass of 4.0026. That was obtained by adding the amount of protons and neutrons in a helium atom together.

On the other hand 4.0026 is also the weight in grams of 1 mole of helium. Since 1 mole contains the exact same number of atoms regardless of what element it is how is this possible?

Did Avogadro carefully plan it out and made sure 1 mole of any element had the exact same weight in grams as the atomic mass unit of any given element?

2. Jul 3, 2007

### chemisttree

Think of moles as a number. Like dozen, couple, gross, etc...

Would a dozen grapefruit weigh as much as a dozen ping pong balls?

The definition of mole is that a certain number (Avogadro's Number) of atoms will weigh a certain amount (atomic weight).

Yes, it is that simple

3. Jul 3, 2007

### Renge Ishyo

>>>>
I can't understand how both the atomic weight of one atom of an element and the weight in grams of a mole of that element are the exact same thing.

For example helium has an atomic mass of 4.0026. That was obtained by adding the amount of protons and neutrons in a helium atom together.

On the other hand 4.0026 is also the weight in grams of 1 mole of helium. Since 1 mole contains the exact same number of atoms regardless of what element it is how is this possible?
>>>>>

They are not the same thing. The mass of one atom of helium is only approximately 6.70x10^-24 g. If you multiply that by avogadros number then you get approximately 4 grams for a mole of helium atoms. One of the main reasons why the mole is used to begin with is that the mass of a single atom is far too small for us to measure out on macroscopic scales.

The "atomic mass units" are ratios between atoms that also happen to be equal to the atomic weight (in grams/mole) of a substance. The difference between the two is that the atomic mass units are dimensionless quantities...meaning that they can be used to multiply atomic weights that are NOT given in grams/mole. Say you have one pound of hydrogen and you wanted to know how many pounds of helium would be equivalent to this same amount of atoms. The answer would be approximately 4 pounds. You can do the calculation with amu units:

(1 pound) (4amu) = 4 pounds Helium

You CANNOT do this calculation if you use the atomic weight of helium in your calcuation because the units don't match up:

(1 pound) (4g/mol) = 4 pound*g/mol = nonsense units

This is why the more general amu is used on the periodic table. Any two equal amounts of atoms (say a mole of each) will give the same amu that appears on the table if you divide them regardless of the units the scientist is using.

Ex.

4 g/mol He/ (1g/mol H) = 4 amu
.004kg/mol He/ (.001 kg/mol H) = 4 amu
.0088lb/mol He/(.0022lb/mol H) = 4 amu

4. Jul 20, 2007

### bhuwana65

can any body explain to me in simple terms "what is amu" and how it is related to at.weights as i have to explain this to group of students of grade X

5. Jul 21, 2007

### Kushal

The unified atomic mass unit (u), or dalton (Da), is a small unit of mass used to express atomic and molecular masses. It is defined to be one twelfth of the mass of an unbound atom of the carbon-12 nuclide, at rest and in its ground state.

from wikipedia......

couldn't be of much greater help...

6. Jul 21, 2007

### Gokul43201

Staff Emeritus
A single nucleon (proton/neutron) weighs about $1.67\cdot10^{-24}g$. The Avogadro/Loschmidt number is about $6.02 \cdot 10^{23}$. So, this many nucleons (a mole of nucleons) will weigh 1.0 gram. A mole of atoms with M nucleons will thus weigh M grams.

Last edited: Jul 21, 2007