Flashcards empirical formula, Slides of Chemistry

Download Flashcards empirical formula and more Slides Chemistry in PDF only on Docsity! Empirical formulae • Work out the formulae of the following compounds: • Sodium sulphide Na2S • Calcium chloride CaCl2 • Aluminium oxide Al3O2 Empirical formulae • Work out the formulae of the following compounds: • Sodium sulphide • Na 2 S • Calcium chloride • CaCl 2 • Aluminium oxide • Al 2 O 3 Working out empirical formulae 1 • Work out the empirical formulae of the following: • C 6 H 12 O 6 • C 2 H 4 • C 6 H 3 N 3 O 6 Working out empirical formulae 1 • Work out the empirical formulae of the following: • C 6 H 12 O 6 • CH 2 O • C 2 H 4 • CH 2 • C 6 H 3 N 3 O 6 • C 2 HNO 2 Working out empirical formulae 2 • The empirical formula can be worked out by using data about % composition or the mass of each element in the compound. • For example: • Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen. Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen. • Write down the symbol for each element • Write down the mass or % composition of each element • Divide by the relative atomic mass (A r ) of each, to find the number of moles. Symbol K O % comp or mass 83 17 A r 39.1 16 n = 2.12 1.06 divide by smallest Lowest ratio Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen. • Write down the symbol for each element • Write down the mass or % composition of each element • Divide by the relative atomic mass (A r ) of each, to find the number of moles. • Divide each of the values by the smallest number of moles of the elements present. Symbol K O % comp or mass 83 17 A r 39.1 16 n = 2.12 1.06 divide by smallest 1.06 1.06 Lowest ratio Calculate the empirical formula of a compound containing 83g of potassium and 17g of oxygen. • Write down the symbol for each element • Write down the mass or % composition of each element • Divide by the relative atomic mass (A r ) of each, to find the number of moles. • Divide each of the values by the smallest number of moles of the elements present. • This will often give you the lowest whole number ratio. Symbol K O % comp or mass 83 17 A r 39.1 16 n = 2.12 1.06 divide by smallest 1.06 1.06 Lowest ratio 2 1 Practice Questions 2. What is the empirical formula of a compound containing 26.45g of aluminium and 23.55g of oxygen? • When you divide through by the smaller of the two values for n, you should get 1:1.5. • Do not round this value. • Instead, you should multiply through by 2 • 1:1.5 = 2:3 • Al 2 O 3 Converting empirical formulae to molecular formulae • What is the empirical formula of a hydrocarbon containing 85.7% carbon? • The same hydrocarbon has a molar mass of 42. What is its molecular formula? Converting empirical formulae to molecular formulae • A hydrocarbon contains carbon and oxygen only, hence the remaining 14.3% must be hydrogen. • The empirical formula is CH 2 Symbol C H % comp or mass 85.7 14.3 A r 12 1 n = 7.14 14.29 divide by smallest 7.14 7.14 Lowest ratio 1 2