Structure 1.4.3 — Molar Mass and the n = m/M Relationship

Molar Mass

The molar mass (denoted \( M \)) is the mass of one mole of a substance. It is expressed in units of grams per mole (g mol^–1).

The molar mass is numerically equal to the relative molecular or formula mass (Mr) but expressed in grams.

For example:

• Oxygen, \( O_2 \): \( M = 32.00 \, \text{g mol}^{-1} \)
• Water, \( H_2O \): \( M = 18.02 \, \text{g mol}^{-1} \)
• Sodium chloride, \( NaCl \): \( M = 58.44 \, \text{g mol}^{-1} \)

Key Formula : Mass, Moles and Molar Mass

The relationship between the mass of a substance, its amount in moles, and molar mass is:

\( n = \frac{m}{M} \)

• \( n \) = number of moles (mol)
• \( m \) = mass of substance (g)
• \( M \) = molar mass (g mol^–1)

This formula allows you to move easily between the mass, the amount in moles, and molar mass of any substance.

 Link to Particles (via Avogadro’s constant):

You can also link mass and particles using:

Number of particles = \( n \times N_A \)

Where \( N_A = 6.022 \times 10^{23} \, \text{mol}^{-1} \) (Avogadro constant)