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