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Cambridge IGCSE chemistry

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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

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

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

  • Iron(II): Fe2+

  • Copper(II): Cu2+

  • Silver(I): Ag+

  • Chromium(III): Cr3+

  • Ammonium: NH4+

  • Hydroxide:  OH

  • Nitrate: NO3

  • Sulfate: SO42-

  • Carbonate: CO32-

  • Hydrogen carbonate: HCO3

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

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