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Cambridge IGCSE chemistry
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States of matterStates of matter3 Topics

Changing state4 Topics

Diffusion4 Topics

Atoms, elements and compoundsElements, compounds and mixtures

Atomic structure and the Periodic Table

Isotopes

Ions and ionic bonds

Simple molecules and covalent bonds

Giant covalent structures

Metallic bonding

StoichiometryFormulas of compounds3 Topics1 Quiz

Chemical equations5 Topics

Relative atomic and molecular mass2 Topics

Moles2 Topics

Calculating masses in reactions3 Topics

Percentage yield and percentage purity2 Topics

Empirical and molecular formulae2 Topics

Moles and concentration2 Topics

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
Al_{2}S_{3}
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
Na_{2}O
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): Fe^{2+}
Copper(II): Cu^{2+}
Silver(I): Ag^{+}
Chromium(III): Cr^{3+}
Ammonium: NH_{4}^{+}
Hydroxide: OH^{–}
Nitrate: NO_{3}^{–}
Sulfate: SO_{4}^{2}
Carbonate: CO_{3}^{2}
Hydrogen carbonate: HCO_{3}^{–}
What is the formula of calcium bromide?
Step 1: Write the formulae of the ions: Ca^{2+ }and Br^{–}
Step 2: Balance the charges. We need two Br^{–} x 2 to balance one Ca^{2+}
Step 3: Write the formula with the metal ion first: CaBr_{2}
What is the formula of aluminium sulfate ?
Step 1: Write the formulae of the ions: Al^{3+ }and SO_{4}^{2}
Step 2: Balance the charges. Al^{3+ }x 2 = +6 and SO_{4}^{2 }x 3 = 6
Step 3: Write the formula with the metal ion first: Al_{2}(SO_{4})_{3}