# Chemistry Unit 1.2 - Formulae, Equations and Moles

Notes on formulae, equations and moles

- Created by: mar-rii
- Created on: 01-01-10 16:51

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Chemistry Unit 1.2: Formulae, Equations and Moles

Equations and Moles

Empirical and Molecular Formulae

The empirical formula of a compound tells us the simplest ratio of atoms in that compound. It is

found from experimental data.

Question

A hydrocarbon was found to contain 75% carbon and 25% hydrogen by mass. Determine its empirical

formula.

C H

Mass(g) 75 25

Divide by the relative atomic mass 75/12 25/1

Ratio of atoms 6.25 25

Divide by smallest 6.25/6.25 25/6.25

. 1 4

So, the empirical formula of this hydrocarbon is CH4.

The molecular formula of a compound tells us the actual number of atoms in a compound of one

mole of that compound.

Question

It was found that a hydrocarbon had an empirical formula of CH3. It was found that its relative formula mass

(r.f.m.) is 30. Determine its molecular formula.

1. Determine r.f.m. of CH3 = (12x1) + (1x3) = 15

2. Determine the ratio of mass = 30:15 = 2:1

3. Multiply the number of atoms by the ratio to give molecular formula = 2 * CH3 =C2H6

Compound Formulae

Find the Compound Formula for AB where

Step 1:

Step 2: If , they combine like this:

Step 3: If not...: cancel down this fraction into its simplest form, like:

Step 4: Call this 'cancelled down' version of x and y as

Step 5: The Compound can be written as

Example:

1.

2. 3 is not equal to 2

3.

4.

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