L4: Reacting Masses

Objectives

• Define relative atomic mass, A_r, as the average mass of naturally occurring atoms of an element on a scale where the ¹²C atom has a mass of exactly 12 units

• Define relative molecular mass, M_r, as the sum of the relative atomic masses (relative formula mass or M_r will be used for ionic compounds)

• Define the mole in terms of a specific number of particles called Avogadro’s constant

When counting the number of atoms, molecules, or formula units in a substance, we are going to be dealing with very BIG numbers.

Let’s take a look at a special big number: 6.02 x 10²³

This number is called Avogadro’s constant. Something very interesting happens if you take 6.02 x 10²³ particles of any substance and measure its mass.

The mole is a unit that tells us the amount of substance there is.

Specifically, it is the number of particles (6.02 x 10²³, Avogadro’s constant) needed for a substance’s mass to equal its relative atomic mass in grams.

One mole of C = 6.02 x 10²³ carbon atom

One mole of H₂ = 6.02 x 10²³ H₂ molecules

One mole of MgO = 6.02 x 10²³ MgO “formula units”