# The Mole & the Molar Mass

## The Mole

### Definition

It is the amount of matter that contains as many objects as the number of atoms in exactly 12 grams of isotopically pure 12C, which contains 6.02 × 1023 carbon atoms.
1 Mole = 6.02 × 1023 called "Avogadro's Number"

### Examples

1 mol 12C atoms = 6.02 × 1023 atoms of 12C
1 mol H2O molecules = 6.02 × 1023 molecules of H2O
1 mol NO3 ions = 6.02 × 1023 nitrate ions

Remarks:
• The reason why 12 grams, it is because the atomic mass of a C atom.
• isotopically-pure means there are 4 different isotopes of C, of which 12C is the most stable and abundant isotope.
2 mole of N2 molecules, should contain 2 × 6.02 × 1023 of N2 molecules.
½ mole of N2 molecules, should contain ½ 6.02 × 1023 of N2 molecules.

### Exercise: Mole & Molar Mass

Exercise on Mole & Molar Mass

Check your answers here: Solution to the Exercise on Mole & Molar Mass

Exercise on Number of Atoms and Moles

Check your answers here: Solution to the Exercise on Number of Atoms and Moles

## The Molar Mass

### Definition:

It is the mass in grams of 1 mol of a substance. It has the unit of g/mol . i.e.
It numerically equals the formula weight (in amu) of the substance.

### Examples

1 atom of 12C has a mass of 12 amu ⇒ 1 mol of 12C has a mass of 12 g
1 atom of 35.5Cl has a mass of 35.5 amu ⇒ 1 mol of 35.5Cl has a mass of 35.5 g
1 atom of H2O has a mass of 18 amu ⇒ 1 mol of H2O has a mass of 18 g
Hence, 1 mole of H2O contains 6.02 × 1023 of H2O
A mass of all these molecules is 18 g

Exercise on Finding Mass and Number of Moles

Check your answers here: Solution to the Exercise on Finding Mass and Number of Moles

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Date of last modification: Summer , 2019