WJEC Chemistry for AS: Study and Rev Guide

1.1 Formulae and equations The formula of a compound is a set of symbols and numbers. The symbols say what elements are present and the numbers give the ratio of the numbers of atoms of the different elements in the compound. E.g. hydrogen sulfur oxygen H 2 SO 4 2 atoms of hydrogen 1 atom of sulfur 4 atoms of oxygen Many compounds consist of molecules in which the atoms are bonded covalently. To show multiple molecules you write the appropriate number in front of the formula. This number multiplies everything after it. E.g. in 3HNO 3 atoms of hydrogen = 3 × 1 = 3 atoms of nitrogen = 3 × 1 = 3 atoms of oxygen = 3 × 3 = 9 Many compounds do not consist of molecules but consist of ions and they form through ionic bonding. For an ionic compound the total number of positive charges must equal the total number of negative charges in one formula unit of the compound. The formula for ionic compounds can be calculated by following these steps: 1 Write the symbols of the ions in the compound. 2 Balance the ions so that the total of the positive ions and negative ions adds to zero. (The compound itself must be neutral.) 3 Write the formula without the charges and put the number of ions of each element as a small number following and below the element symbol. (Use brackets to denote more than one compound ion.) Example 1 Sodium oxide 1 Sodium is in group 1, oxygen is in group 6, therefore the ions are Na + and O 2– . 2 Two Na + ions are needed to balance the charge on one O 2– ion to make the total charge zero (+1 +1 –2 = 0). 3 Formula is Na 2 O (the ‘1’ does not need to be included). Example 2 Calcium hydroxide 1 Calcium is in group 2, hydroxide is a compound ion, the ions are Ca 2+ and OH – . 2 Two OH – ions are needed to balance the charge on one Ca 2+ ion to make the total charge zero (–1 –1 +2 = 0). 3 Formula is Ca(OH) 2 (note the use of a bracket around the OH before adding the 2). Formulae of compounds and ions Grade boost Learn the formulae of the common acids and common gases. QUICKFIRE ICKFIRE ICKFIRE  What is the formula of: a) Hydrochloric acid? b) Sulfuric acid? c) Ammonia? d) Methane? QUICKFIRE ICKFIRE ICKFIRE  How many atoms of each element are present in 2CH 3 CO 2 H? QUICKFIRE ICKFIRE ICKFIRE  How many atoms of oxygen are present in Ca(HCO 3 ) 2 ? Grade boost Learn the formula for Group 1, 2, 6 and 7 ions. Pointer You do not need to learn the formula for ionic compounds. QUICKFIRE  Write the formula for: a) Sodium carbonate b) Barium sulfate c) Ammonium sulfate. 8 AS Chemistry: Study and Revision Guide