Lesson 5 – types of formula

Learning outcomes• Explain the terms empirical formula and molecular formula.

• Calculate empirical and molecular formulae.

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Empirical formula

• Simplest way of showing a formula

• Sodium chloride is a giant structure, millions of ions are bonded together, it would be very hard to represent that as a formula.

Finding the empirical formula

• We can find the % of each element in a compound by experiments

• Then we can work out the simplest whole number ratio for each type of atom in the compound

• This is called its EMPIRICAL FORMULA

Method

1. list all the elements in the compound2. underneath each element, write the

experimental masses or percentages3. divide each mass or percentage by the Ar

for that element4. turn those numbers into a ratio by

multiplying or dividing by a well chosen number (e.g. The smallest number!)

5. get the ratio into its simplest form, this is the empirical formula of the compound

Worked exampleFind the empirical formula of the iron oxide producedwhen 44.8g of iron reacts with 19.2g of oxygen. (Ar Fe = 56, Ar O = 16)

 MethodList the two elements Fe OWrite in the masses 44.8 19.2Divide by the Ar of each element 44.8/56 19.2/16

0.8 1.2Multiply by 10 (to make whole number) 8 12Work out what 8 and 12 are divisible by (hint 4 is the magic number!) 2 3

So the simplest formula is Fe2O3

Next steps...

• Complete the worksheet• Complete question 1 in the text book

• Follow the steps for success!

• If you finish, extra questions are available

Molecular formula

• Molecular formula are used for compounds that exist as simple molecules.

• A molecular formula tells you the number of each type of atom that make up a molecule.

Week 2

© Pearson Education Ltd 2008This document may have been altered from the original

Molecule of propane

What’s the molecular formula of propane?

Week 2

© Pearson Education Ltd 2008This document may have been altered from the original

A molecule of butane

What’s the molecular formula of butane?

What would be the empirical formula?

Worked example

A compound has an empirical formula of CH2 and a relative molecular mass Mr of 56.0. What is its molecular formula?

Empirical formula mass of CH2 = 12.0 + (1.0x2) = 14.0

Number of CH2 units in a molecule = 56.0/14.0 = 4

Molecular formula (4 x CH2) = C4H8

• Complete question 2 in the text book

Quiz quiz trade

• Write a question that either requires someone to calculate an empirical formula or a molecular formula

• Calculate the answer to your question

• Move around the room, ask a question, answer a question, swap cards