Moles

The mole is a unit of measure in chemistry.  Let's look at other units of measure that you are familiar with first- for example, a dozen.  We all know that a dozen = 12 of something.  What about a gross?  A gross is equal to 12 dozen or 144 of something.  How about a ream?  Paper is sold in reams which are equal to 500 sheets.  So what is a mole equal to?  A mole is 6.022 x 10²³ somethings, where those somethings could be atoms, molecules, or ions.  For example, a mole of calcium contains 6.022 x 10²³ atoms.  A mole of water contains 6.022 x 10²³ molecules.

The weight of one mole of a substance is defined as its molar mass.  Even though all moles have 6.022 x 10²³ particles, they do not have the same masses.  This is because each atom within a substance has a different mass as seen in the example below.

How do you determine molar mass?  You use the atomic masses of each element on the periodic table and you add them together to get the molar mass.

Examples: 

1. Helium has the formula He so if we find helium on the periodic table we find that its mass is 4.0026 g/mol so the molar mass is thus equal to 4.0026 g/mol.

2.  Fluorine exists as a diatomic molecule so its formula is F₂.  When we find fluorine on the periodic table we find that its atomic mass is 18.998 g/mol.  That is the mass of one atom but fluorine is diatomic we need to multiply by two to get the molar mass of F₂.  This gives us an answer of 37.996 g/mol.

3. Water has a formula of H₂O.  To find its molar mass, we need to find the atomic mass of hydrogen (1.008 g/mol), then multiply it by two since there are two H's in the formula (2.016 g/mol) and then add it to the mass of one oxygen (15.9994 g/mol).  This gives us a molar mass of 18.015 or approximately 18 g/mol.