10.03.02 Relative Formula Mass

Syllabus

- The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula.
- In a balanced equation, the sum of the relative formula masses of the reactants in the quantities shown equals the sum of the relative formula masses of the products in the quantities shown.

What does this mean?

Why must we balance equations?

We know how to write a simple word equation

Hydrogen + Oxygen --> Water

And we've seen that these can be be written as symbol equations.

H2(g) + O2(g) --> H2O(l)

But this isn't balanced. So, we to double the water and Hydrogen to balance the H's and O's.

2 H2(g) + O2(g) --> 2 H2O(l)

But these numbers aren't masses.

It does not mean that 2g of Hydrogen reacts with 1g of Oxygen to make 2g of water because that isn't possible because mass in a reaction must be conserved (not made or destroyed)

In fact, 4g of Hydrogen reacts with 32g of Oxygen to make 36g of water.

The numbers in the equation (coefficients) mean that 'two lots' of Hydrogen react with 'one lot' of Oxygen to make 'two lots' of water.

So,

Two hundred Hydrogen molecules would react with one hundred Oxygen molecules making 2 hundred water molecules.

Or,

Two million Hydrogen molecules would react with one million Oxygen molecules to make 2 million water molecules.

But molecules and atoms are so small we need a number even higher than trillions.

We call that number "the mole".

To make Hydrogen and Oxygen react completely, we should always use twice as many moles of Hydrogen than the number of moles of Oxygen

How do we work out the number of moles of an element?

First we must know how much a mole of a substance weighs (the Molar Mass or Relative Formula Mass Mr).

On the Periodic Table each element has a Mass Number (Relative Atomic Mass or Ar).

This is the mass of one mole of individual atoms (in g/mol)

So, the Mass number is the Molar Mass of a Noble Gas since Noble Gases exist as individual atoms.

Metals (and also Carbon and Silicon) exist in giant structures. We don't know how many metal atoms are in these giant structures so we also use the formula Fe for Iron or C for Carbon

Their Mass Numbers are their Molar Masses.