Relative atomic mass Calculating number of moles

1- Relative masses:

Materials react with each other in certain ratios.

a- Relative atomic masses (Ar) in grams: the average of the masses of the isotopes in a naturally occurring sample of the element relative to the mass of 1/12 of an atom of carbon-12.

E.g:

silver atomic mass is  107.87 that is equal to relative atomic mass of 107Ag and 109Ag

b- Relative molecular mass (Mr) in grams: is the sum of the relative atomic masses of the individual atoms making up a molecule.

Example 1: relative molecular mass of methane (CH4)=

12.01 (Ar of C) + (4x1.01 (Ar of H)) = 16.05g

Example 2:

Relative atomic mass of ethanoic acid or acetic acid (CH3COOH)=

12.01 + (3x1.01)+ 12.01 +(2x16) +1.01= 60.06g

Mole: is the amount of substance that contains the same number of particles (atoms, ions, molecules, etc.) as there are carbon atoms in 12 g of carbon-12.

This number is called Avogadro’s constant “L” (or NA) .

Avogadro’s no. has the value 6.02 × 1023 mol−1. So, 12.00 g of carbon-12 contains 6.02 × 1023 carbon atoms.

- The meaning of average atomic mass:

The Ar of oxygen is 16.00, which means that, on average, each oxygen atom is 16 12 times as heavy as a carbon-12 atom. Therefore 16 g of oxygen atoms must contain the same number of atoms as 12 g of carbon-12, i.e. one mole, or 6.02 × 1023 atoms.

Mass of one mole = the sum of grams of all atoms of the molecule.

Example: the mass of one mole of water H2O=

(2x1.01) + 16 = 18.02grams = 6.02x1023molecules of water.

This means; mass of one molecule of water = "18.02" /(6.02x 10^23 ) = 2.99x10-23g

Remember: mass of molecule is very small compared to mass of one mole which is greater than 1.