Chemistry 103 - The Mole

Mole (mol)
    The mole is the SI unit for amount of a substance.

    The mole is the quantity of a substance which contains as many individual units (e.g. moleucles,
    formula units, atoms) as there are atoms in exactly 0.012 kg (12 g) of carbon-12.

    The number of units in a mole is 6.022 x1023 and is often referred to as Avogadro's number.

    One mole of carbon-12 contains 6.022 x1023  atoms of carbon-12.

    One mole of water contains 6.022 x1023  molecules of water.

    The mole is defined so that a sample of an element with a mass equal to its atomic mass in
    grams contains Avogadro's number of that element.

    There are 22.99 g of sodium in one mole of sodium metal.  There are 10.81 g of boron in one
    mole of elemental boron.

    The mass of one mole in grams is often called the molar mass.

EXAMPLE 1
How many moles of Na are there in a 11.495 gram sample?        ( 0.5000 mol Na)
 
 
 
 
 
 
 
 


Molar Mass of Compounds

The molar mass of a compound is equal to the sum of the atomic masses of the atoms
forming the compound.

EXAMPLE 2
What is the molar mass of H2O?        (18.02 g/mol )
 
 
 
 
 
 
 
 

EXAMPLE 3
How many grams of water are contained in a 19.70 mol sample?    (355.0 g H2O)
 
 
 
 
 
 
 
 


Hydrates
A hydrate is a crystalline compound with water weakly bound within the crystal. The water in the
compound is referred to as waters of hydration.

Some examples of hydrates are:

    CuSO4.5 H2O   Copper(II) sulfate pentahydrate
    AlK(SO4)2.12 H2O  Aluminum potassium sulfate dodecahydrate

When determining the formula mass for a hydrate the waters of hydration must be included.

EXAMPLE 4
What is the formula mass of copper(II) sulfate pentahydrate, CuSO4.5 H2O?        (249.72 g/mol)