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Empirical Formula

Molecular Formula A molecular formula shows the elements that a compound is made out of and the number of atoms of each element present in one molecule of the compound. The numbers are shown written as subscripts. e.g. The molecular formula of butane is C 4 H 10

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Empirical Formula

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  1. Molecular Formula A molecular formula shows the elements that a compound is made out of and the number of atoms of each element present in one molecule of the compound. The numbers are shown written as subscripts. e.g. The molecular formula of butane is C4H10 One molecule of ethane contains 4 carbon atoms and 10 hydrogen atoms. The molecular formula of water is H2O One molecule of water contains 2 hydrogen atoms and one oxygen atom. Empirical Formula The empirical formula gives the simplest whole number ratio of each type of atom in a compound e.g. The molecular formula of butane is C4H10 The empirical formula of butane is C2H5

  2. Relative molecular mass Molecules are incredibly light so when discussing how heavy a molecule is Chemists often find the relative molecular mass. This is measured in atomic mass units. The table below shows the relative atomic masses of various elements which can be used to calculate the relative molecular mass of molecules. e.g. The molecular formula of butane is C4H10 The relative molecular mass of ethane is 4 x 12 + 10 x 1 = 58 atomic mass units Element Symbol Relative Atomic Mass Hydrogen H 1 Carbon C 12 Oxygen O 16 Nitrogen N 14 Calcium Ca 40 Silicon Si 28

  3. Elemental Analysis Is a technique that allows a chemist to workout (as a percentage) how much each element contributes to the total mass of the molecule. If you know these percentages you can work out what the empirical formula of a molecule so it can help Chemists work out what they have made. Where's the maths? Ratio Percentages Fractions Substitution

  4. Use your skills with fractions, percentages, ratios and substitution to work out missing information about molecules and even identify mystery compounds. Use the table below to help you. Element Symbol Relative Atomic Mass Hydrogen H 1 Carbon C 12 Oxygen O 16 Nitrogen N 14 Calcium Ca 40 Silicon Si 28

  5. There a few different ways of using this resource: • Fill in the gaps in the table (possibly in pairs- each use a different colour) • Cut up the completed grid and use it as a card sort (possibly only use one slide rather than two) • Print the reversed complete grid with a picture on the back of the page before you cut it out into cards. Put the cards in the correct places on the template grid. Flip over each card to reveal the picture (and hence check the cards are in the correct place.

  6. Fill in gaps (given formula)

  7. answers

  8. Fill in gaps (harder)

  9. answers

  10. Card sort

  11. Card sort with template (given formula)

  12. (insert picture of your own choice)

  13. (insert picture of your own choice)

  14. Card sort with template (harder)

  15. (insert picture of your own choice)

  16. (insert picture of your own choice)

  17. Alternative ratio activity

  18. Match the following molecules that contain the same ratio of elements. CH2O C2H2 C6H12 C5H10 H2O C2H4O2 C4H8 C6H12O6 C2H6O H2O2 C5H8O C2H4 C6H12O3 C6H6 C3H6O3 The simplest ratio of elements in a compound is called the empirical formula can you find the empirical formula for each compound?

  19. Extension • The mass of each of the compound is shown on the next slide. • Can you work out the mass of the following atoms H (hydrogen) C (carbon O (oxygen)

  20. CH2O 30 C2H2 26 C6H12 84 C5H10 70 H2O 18 C2H4O2 60 C4H8 56 C6H12O6180 C2H6O 46 CH416 C2H428 C5H8O 84 C6H12O3 132 C6H678 C3H6O3 90

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