Introduction to MOLES

Introducing 'moles' and their the connection with mass and formula mass

(a) WHAT IS THE MOLE CONCEPT? and WHAT IS ONE MOLE OF A SUBSTANCE?

The 2nd part of the heading is 'easy', the first part is a bit more 'abstract' to get your head round!

The mole concept is an invaluable way of solving many quantitative problems in chemistry!

Its a very important way of doing chemical calculations!

The theoretical basis is explained in section (b).

The mole is most simply expressed as the relative 'formula mass in g' or the 'molecular mass in g' of the defined chemical 'species', and that is how it is used in most chemical calculations. The mass of one mole of a substance is sometimes referred to as the molar mass.

The atomic/formula mass in grams = one mole of the defined substance.

If your are dealing with individual atoms, one mole of equals the relative atomic mass in grams.

This can be expressed as a simple formula ...

moles of species = (actual mass of species in g) / (atomic/formula mass of species)

therefore (using triangle on right if necessary)

 mass of species in g = moles species x atomic/formula mass of species

atomic/formula mass of species = mass of species in g / moles of species

Note these equations are for either an element or a compound,

but, whatever, you must clearly define the chemical species you mean for any mole calculation e.g.

Al metal element atom, H₂O covalent molecule, an element O₂ molecule, Na⁺Cl^- ionic compound or just any compound formula like CuSO₄ etc. etc.

This specificity cannot be overemphasised.

 

M_r is 'shorthand' for relative formula mass or molecular mass in amu (atomic mass units) and you must be able to work these out correctly from a given formula (Calculating relative formula/molecular mass of a compound or element molecule).

The term relative molecular mass (sum of the atomic masses of the atoms in a single molecule of the substance) is usually applied to definite molecular species.

Using the following atomic masses: H = 1, O = 16, N = 14, C = 12, Na = 23, Cl = 35.5, S = 32

and the three formulae above relating moles, mass and formula mass ...

molecular mass 18 for the water molecule H₂O, 17 for the ammonia molecule NH₃

so 1 mole of water is 18g, 0.333 mole = 0.333 x 18 = 6g

for ammonia 1mol = 17g, 34g = 34/17 = 2 mols ammonia

 

16 for the methane molecule CH₄ and 180 for the glucose sugar molecule C₆H₁₂O₆

so 0.5 mol methane = 0.5 x 16 8g, 72g = 72/16 = 4.5 mols methane

for glucose 18g = 18/180 = 0.10 moles, 0.05 mole = 0.05 x 180 = 9.0g glucose

the element nitrogen consists of N₂ molecules, molar mass = 28g, 0.25 moles = 0.25 x 28 = 7.0g

Relative atomic mass of iron is 56, 7g = 7/56 = 0.125 mol Fe (Relative atomic mass explained)

So, these calculations are quite simple, but they are often just one part of solving a more complex problem.

 

The term relative formula mass (sum of the atomic masses of the atoms in a specified formula) can be used for ANY specified formula of ANY chemical substance, though it is most often applied to ionic substances.

e.g. mass of 1 mole of ionic sodium chloride NaCl or Na⁺Cl^- is 58.5g (from 23 + 35.5)

mass of 1 mole of ammonium sulfate (ionic salt) (NH₄)₂SO₄ or (NH₄⁺)₂(SO₄^2-) =  130g