# 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 ^{12}C,
which contains 6.02 × 10^{23} carbon atoms.

1 Mole = 6.02 × 10^{23} called "Avogadro's Number"

### Examples

1 mol ^{12}C atoms = 6.02 × 10^{23} atoms of ^{12}C

1 mol H_{2}O molecules = 6.02 × 10^{23} molecules of H_{2}O

1 mol NO_{3}^{− } ions = 6.02 × 10^{23} 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
^{12}C is the most stable and abundant isotope.

2 mole of N_{2} molecules, should contain 2 × 6.02 × 10^{23} of N_{2} molecules.

½ mole of N_{2} molecules, should contain ½ 6.02 × 10^{23} of N_{2} 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 ^{12}C has a mass of 12 amu ⇒ 1 mol of ^{12}C has a mass of 12 g

1 atom of ^{35.5}Cl has a mass of 35.5 amu ⇒ 1 mol of ^{35.5}Cl has a mass of 35.5 g

1 atom of H_{2}O has a mass of 18 amu ⇒ 1 mol of H_{2}O has a mass of 18 g

Hence, 1 mole of H_{2}O contains 6.02 × 10^{23} of H_{2}O

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