Lesson 5 – types of formula

Learning outcomes Explain the terms empirical formula and molecular formula. Calculate empirical and molecular formulae. © Pearson Education Ltd 2008 This document may have been altered from the original

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 compound 2.underneath each element, write the experimental masses or percentages 3.divide each mass or percentage by the A r for that element 4.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 example Find the empirical formula of the iron oxide produced when 44.8g of iron reacts with 19.2g of oxygen. (A r Fe = 56, A r O = 16) Method List the two elements Fe O Write in the masses Divide by the A r of each element 44.8/ / Multiply by 10 (to make whole number) 8 12 Work out what 8 and 12 are divisible by (hint 4 is the magic number!) 2 3 So the simplest formula is Fe 2 O 3

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 2008 This document may have been altered from the original Molecule of propane What’s the molecular formula of propane?

Week 2 © Pearson Education Ltd 2008 This 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 CH 2 and a relative molecular mass M r of What is its molecular formula? Empirical formula mass of CH 2 = (1.0x2) = 14.0 Number of CH 2 units in a molecule = 56.0/14.0 = 4 Molecular formula (4 x CH 2 ) = C 4 H 8

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