[EX5TASY]

I feel really stupid, because this seems like basic stuff. I'm confused on how Grams is interpreted from the Periodic Table. In my book it says to consider the masses for each element as an AMU or a Dalton, but then in later examples, it changes those directly to Grams. Is there something i'm missing here?

For example, a stoichiometric problem:

Ethanol, C₂H₅OH. Calculating molar mass:

Mass of C = 2 mol  x  12.011 g/mol  = 24.022g

I understand the setup, but where does grams come from? Any help is greatly appreciated!

[english]

Atomic masses are based on amu's.  1 amu = 1/12 of ¹²C.

The atomic mass of each element is based on the % abundance of each naturally occurring isotope of that element, whose masses are defined by amu.

For example, ¹³C accounts for about 1.11% of naturally occurring carbon.



Each atomic mass of each element in grams = 1 mol of that element.


So carbon's calculated atomic mass is 12.01 amu.  So 12.01 g = 1 mol of carbon atoms.

[Yggdrasil]

Avogadro's number is such an important constant in chemistry because it is the conversion factor from AMU to grams:

6.02x10²³ AMU = 1 g

This is also why a mole is a convenient quantity to work with.  If a molecule has a mass of x AMU, then a mole of that molecule will have a mass of x g. 

For example, take k.V.'s example of ¹²C.  A single atom of ¹²C has a mass of 12 amu.  How much mass will a mole of ¹²C atoms have?

6.02x10²³ * 12 amu * (1g / 6.02x10²³ amu) = 12 g

So a mole of ¹²C will have a mass of 12g.

[english]

Avagadro's number is also defined on ¹²C.

number of atoms in 12g of ¹²C = 6.0223 x 10²³ atoms.


There are 6.0223 x 10²³ Si atoms, O atoms, N atoms, Fe atoms, Sc atoms, etc., in 12g of ¹²C.  This is conveniently called a mole, as there are also 6.0223 x 10²³ molecules of carbon dioxide, hydrogen fluoride, etc., as well as 6.0223 x 10²³ formula units of NaCl, MgCl₂, etc in 12g of ¹²C.

Strange how this number of all of these varying sizes of compounds, ionic solids, and atoms fit into 12g of this one atom, but we owe this wonderul constant to the Italian physicist Amedeo Avagadro.


We need those physicists for something.