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Cambridge IGCSE chemistry

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  1. States of matter
    States of matter
    3 Topics
  2. Changing state
    4 Topics
  3. Diffusion
    4 Topics
  4. Atoms, elements and compounds
    Elements, compounds and mixtures
  5. Atomic structure and the Periodic Table
  6. Isotopes
  7. Ions and ionic bonds
  8. Simple molecules and covalent bonds
  9. Giant covalent structures
  10. Metallic bonding
  11. Stoichiometry
    Formulas of compounds
    3 Topics
    |
    1 Quiz
  12. Chemical equations
    5 Topics
  13. Relative atomic and molecular mass
    2 Topics
  14. Moles
    2 Topics
  15. Calculating masses in reactions
    3 Topics
  16. Percentage yield and percentage purity
    2 Topics
  17. Empirical and molecular formulae
    2 Topics
  18. Moles and concentration
    2 Topics
  19. Calculating gas volumes

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Cambridge IGCSE chemistry Formulas of compounds Deducing formulae
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Lesson 11, Topic 3
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Deducing formulae

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Combining power

  • We can use the idea of combing power with hydrogen (valency) of different atoms to work out the formula

Example 1

What is the formula of aluminium sulfide?

Step 1: Put the combining power beneath each symbol

Al     S

3      2

Step 2: Swap the numbers

Al2S3

Example 2

What is the formula of sodium oxide?

Step 1: Put the combining power beneath each symbol

Na     O

1        2

Step 2: Swap the numbers

Na2O

Deducing formulae from diagrams

  • For simple molecules simply count up the number of atoms

  • For ionic structures containing simple ions: count up each ion and then cancel to the lowest ratio

Deducing formulae of ionic compounds

  • You can write the formula of an ionic compound if you know the charge on the ions

  • You can predict the charge on an ion based on the group in which it is found 

  • The charge on a transition metal is indicated by roman numerals

Examples of transition metal ions

  • Iron(II): Fe2+

  • Copper(II): Cu2+

  • Silver(I): Ag+

  • Chromium(III): Cr3+

Examples of molecular ions

  • Ammonium: NH4+

  • Hydroxide:  OH–

  • Nitrate: NO3–

  • Sulfate: SO42-

  • Carbonate: CO32-

  • Hydrogen carbonate: HCO3–

Example 1

What is the formula of calcium bromide?

Step 1: Write the formulae of the ions: Ca2+ and Br–

Step 2: Balance the charges. We need two Br– x 2 to balance one Ca2+

Step 3: Write the formula with the metal ion first: CaBr2

Example 2

What is the formula of aluminium sulfate ?

Step 1: Write the formulae of the ions: Al3+ and SO42-

Step 2: Balance the charges. Al3+  x 2 = +6 and SO42- x 3 = -6

Step 3: Write the formula with the metal ion first: Al2(SO4)3