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Cambridge IGCSE chemistry
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States of matterStates of matter3 Topics
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Changing state4 Topics
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Diffusion4 Topics
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Atoms, elements and compoundsElements, compounds and mixtures
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Atomic structure and the Periodic Table
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Isotopes
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Ions and ionic bonds
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Simple molecules and covalent bonds
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Giant covalent structures
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Metallic bonding
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StoichiometryFormulas of compounds3 Topics|1 Quiz
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Chemical equations5 Topics
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Relative atomic and molecular mass2 Topics
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Moles2 Topics
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Calculating masses in reactions3 Topics
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Percentage yield and percentage purity2 Topics
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Empirical and molecular formulae2 Topics
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Moles and concentration2 Topics
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Calculating gas volumes
Quizzes
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