Chemistry_Grade_10-12 (1).pdf

Worked example 11 formulae of covalent compounds

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chemical formula of a compound. Worked Example 11: Formulae of covalent compounds Question: Write the chemical formula for water Answer Step 1 : Write down the elements that make up the compound. A molecule of water contains the elements hydrogen and oxygen . Step 2 : Determine the valency of each element The valency of hydrogen is 1 and the valency of oxygen is 2. This means that oxygen can form two bonds with other elements and each of the hydrogen atoms can form one. 78
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CHAPTER 4. ATOMIC COMBINATIONS - GRADE 11 4.9 Step 3 : Write the chemical formula Using the valencies of hydrogen and oxygen, we know that in a single water molecule, two hydrogen atoms will combine with one oxygen atom. The chemical formula for water is therefore: H 2 O . Worked Example 12: Formulae of covalent compounds Question: Write the chemical formula for magnesium oxide Answer Step 1 : Write down the elements that make up the compound. A molecule of magnesium oxide contains the elements magnesium and oxygen . Step 2 : Determine the valency of each element The valency of magnesium is 2, while the valency of oxygen is also 2. In a molecule of magnesium oxide, one atom of magnesium will combine with one atom of oxygen. Step 3 : Write the chemical formula The chemical formula for magnesium oxide is therefore: MgO Worked Example 13: Formulae of covalent compounds Question: Write the chemical formula for copper (II) chloride. Answer Step 1 : Write down the elements that make up the compound. A molecule of copper (II) chloride contains the elements copper and chlorine . Step 2 : Determine the valency of each element The valency of copper is 2, while the valency of chlorine is 1. In a molecule of copper (II) chloride, two atoms of chlorine will combine with one atom of copper. Step 3 : Write the chemical formula The chemical formula for copper (II) chloride is therefore: CuCl 2 79
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4.9 CHAPTER 4. ATOMIC COMBINATIONS - GRADE 11 4.9.2 The formulae of ionic compounds The overall charge of an ionic compound will always be zero and so the negative and positive charge must be the same size. We can use this information to work out what the chemical formula of an ionic compound is if we know the charge on the individual ions. In the case of NaCl for example, the charge on the sodium is +1 and the charge on the chlorine is -1. The charges balance (+1-1=0) and therefore the ionic compound is neutral. In MgO, magnesium has a charge of +2 and oxygen has a charge of -2. Again, the charges balance and the compound is neutral. Positive ions are called cations and negative ions are called anions . Some ions are made up of groups of atoms, and these are called compound ions . It is a good idea to learn the compound ions that are shown in table 4.2 Table 4.2: Table showing common compound ions and their formulae Name of compound ion formula Carbonate CO 3 2 sulphate SO 4 2 Hydroxide OH Ammonium NH 4 + Nitrate NO 3 Hydrogen carbonate HCO 3 Phosphate PO 4 3 Chlorate ClO 3 Cyanide CN Chromate CrO 4 2 Permanganate MnO 4
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