FormulaeCambridge IGCSE Chemistry: Flashcards

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Define the molecular formula of a compound.

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Define the molecular formula of a compound.: The molecular formula shows the number and type of different atoms present in one molecule of a compound.
What is the empirical formula of a compound?: The empirical formula is the simplest whole number ratio of different atoms or ions present in a compound.
How do you deduce the molecular formula from a model or diagram?: Count the number and type of each atom present in one complete molecule shown in the model or diagram. This gives you the molecular formula directly.
What is the difference between molecular formula and empirical formula?: Molecular formula shows the actual number of each atom in one molecule. Empirical formula shows the simplest whole number ratio of atoms. For example, C₆H₁₂O₆ (molecular) has an empirical formula of CH₂O.
How do you construct a word equation?: Write the names of all reactants on the left side, then an arrow, then the names of all products on the right side. For example: Copper + Oxygen → Copper oxide
What are state symbols and what do they represent?: State symbols show the physical state of substances in an equation. (s) = solid, (l) = liquid, (g) = gas, (aq) = aqueous solution.
How do you construct a balanced symbol equation with state symbols?: Write the formula of each reactant and product, then balance the equation by adjusting coefficients so the number of each atom is equal on both sides. Finally, add state symbols in brackets after each substance.
How do you deduce the formula of an ionic compound from ion charges?: Combine the ions in a ratio so that the total positive charge equals the total negative charge. For example, Ca²⁺ and Cl⁻ combine as CaCl₂ because you need two Cl⁻ ions to balance one Ca²⁺ ion.
How do you deduce the formula of an ionic compound from the number of ions in a model or diagram?: Count the number of each type of ion present and write them in the same ratio as they appear. For example, if there are 2 sodium ions and 1 chloride ion shown, the formula is NaCl.
What is an ionic equation?: An ionic equation shows only the ions and molecules that actually participate in a chemical reaction, omitting spectator ions that do not change.
What information do you need to deduce a symbol equation for a chemical reaction?: You need: the names or formulae of the reactants, the names or formulae of the products, and the physical states of all substances involved.
Name the formula of sodium chloride.: NaCl
Name the formula of magnesium oxide.: MgO
Name the formula of calcium carbonate.: CaCO₃
Explain why the molecular formula of a compound can be different from its empirical formula.: The molecular formula is the actual number of atoms in one molecule, whilst the empirical formula is the simplest whole number ratio. Molecular formula can be a simple multiple of the empirical formula. For example, ethane C₂H₆ has an empirical formula of CH₃.