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1 Chapter 3: Chemical Formulae and Equations 1. Relative atomic mass, A r - The relative atomic mass of an element is the average mass of one atom of an element when compared with mass of an atom of carbon-12 1 of the Relative molecular mass, M r - The relative molecular mass of a molecule is the average mass of the one molecule when compared with an atom of carbon-12 1 of the mass of Example: The RAM of Oxygen atom is 16. This means that one atom of oxygen is 16 times heavier than 1 of one atom of 12 carbon Element Symbol Relative Atomic Mass Hydrogen Carbon Nitrogen Sodium Magnesium Aluminium Potassium Calsium Copper Bromine - - The relative molecular mass of a molecule can be calculated by adding up the relative atomic masses of all the atoms that are present in the molecule - Example: H 2 = 2 x A r of H = 2 x 1 = 2 NH 3 = A r of N + 3(A r of H) = (1) = 17 Element Molecular Formula Hydrogen H 2 Oxygen O 2 Chlorine Cl 2 Phosphorus P 4 Sulphur S 8 Ammonia NH 3 Water H 2 O Salt NaCl Relative molecular mass 3. Mole - A mole is an amount of substance that contains as many particles as the number of atoms in exactly 12g of carbon Symbol: mol 4. Avogadro constant or Avogadro number - The Avogadro constant, N A is defined as the number of particles in one mole of a substance - One mole of substance contains 6.02 x particles x mol -1 is known as the Avogadro constant, N A - One mol of atom of any element or compound has the same number of particles, which is 6.02 x particles. - For ionic compounds such as sodium chloride, NaCl, 1 mol of the compound has 1 mol of Na + and 1 mol of Cl -. Thus I mol of NaCl has 2 mol of ions which is 2 x 6.02 x

2 1 mol of oxygen atom,o 1 mol of nitrogen atom, N 1 mol of bromine atom, Br 1 mol of hydrogen atom, H 6.02 x atoms 1 mol of oxygen molecule, O 2 1 mol of nitrogen molecule, N 2 1 mol of bromine molecule, Br 2 1 mol of hydrogen molecule, H x molecules or 2 x 6.02 x atoms - - Examples: A closed glass bottle contains 0.5 mol of oxygen gas, O 2. a) How many oxygen molecules, O 2 are there in the bottle? a) The number of oxygen molecules, O 2 in 0.5 mol of gas = 0.5 mol x 6.02 x mol -1 = 3.01 x Molar mass b) How many oxygen atoms are there in the bottle? b) Each oxygen molecule, O 2 consists of two oxygen atoms Therefore, the number of oxygen atoms in 0.5 mol of gas = 0.5 mol x 6.02 x mol -1 x 2 = 3.01 x x 2 = 6.02 x The molar mass of a substance is the mass of 1 mol of the substance with the unit grams per mol or g mol The mass of one mole of atoms is numerically equal to its relative atomic mass in grams - The mass of one mole of molecules is numerically equal to its relative molecular mass in grams

3 Examples: What is the mass of a) 0.1 mol of magnesium? b) x atoms of magnesium? a) The molar mass of Mg = 24 g mol -1 b) The number of moles of Mg atoms Therefore, the mass of 0.1 mol of Mg = mol 0.4mol = 0.1 mol x 24 g mol -1 = 0.4 mol = 2.4 g The mass of x atoms of Mg = 0.4 mol x 24 g mol -1 = 9.6 g c) How many moles of molecules are there in 16 g of sulphur dioxide gas, SO 2? RMM of SO 2 = (16) = 64 The number if moles of molecules in 16g of SO 2 16g = 0.25mol 1 64gmol 6. Molar volume - The molar volume of a gas is the volume occupied by one mole of the gas - One mole of any gas always has the same volume under the same temperature and pressure. - One mole of a gas occupies a volume of 24dm 3 at room conditions and 22.4 dm 3 at STP Example: a) What is the volume of 1.2 mol of ammonia gas, NH 3 at STP? = 1.2 mol x 22.4 dm 3 mol -1 = dm 3 STP: The condition 0 0 C and 1 atm are called the standard temperature and pressure, STP Room Condition: The condition 25 0 C and 1 atm are called room condition b) How many moles of ammonia gas, NH 3 are present in 600 cm 3 of the gas measured at room conditions? 3 0.6dm = 0.025mol dm mol

4 7. Numerical problems involving gases Example 1: What is the volume of 12.8 g of oxygen gas, O 2 in cm 3 at STP? Mass Moles Volume 12.8?? The number of moles of 12.8 g O g = 0.4mol 1 2(16) gmol The volume of O 2 at STP = 0.4 mol x 22.4 dm 3 mol -1 = 8.96 dm 3 = 8960 cm 3 Example 2: How many molecules of carbon dioxide, CO 2 are produced when 120 cm 3 of the gas is released during a chemical reaction between an acid and a carbonate at room condition? Volume Moles Molecules 120 cm 3?? Volume of CO 2 = 123 cm 3 = dm 3 cm 3 The number of moles of 0.12 dm 3 CO dm = 0.005mol dm mol 0.12dm 3 The number of molecules of CO 2 = mol x 6.02 x mol -1 = 3.01 x Chemical formula - A chemical formula is a representation of a chemical substance using letters for atoms and subscript numbers to show the numbers of each type of atoms that are present in the substance. - Example: H 2, H 2 O, NaCl, Mg(No 3 ) 2,

5 9. Empirical formula - The empirical formula of a compound gives the simplest whole number ration of atoms of each element in the compound. - Example: glucose C 6 H 12 O 6. The ration of carbon to hydrogen to oxygen atoms in the molecule is 6 : 12 : 6. - This can be simplified as 1 : 2 : 1. Therefore, the empirical formula of glucose is CH 2 O. - A sample of aluminium oxide contains 1.08g of aluminium and 0.96g of oxygen. What is the empirical formula of this compound? [RAM: O, 16; Al, 27] Element Al O Mass of element (g) Write down the mass of each element Number of moles Divide each mass by its RAM Ration of moles Divide each number by the smallest number, 0.04 Simplest ration of Multiply each answer by 2 to obtain the moles simplest ration in whole number 2 mol of aluminium atoms combine with 3 mol of oxygen atoms. Therefore, the empirical formula of the compound is Al 2 O 3. There are two ways to determine the empirical formula of metal oxide in laboratory i. Combustion by using crucible lid - Oxide of metal which are more reactive than hydrogen, H 2, in the reactivity series of metals - Example: MgO, ZnO ii. Combustion by using combustion tube - Oxide of metal which are less reactive than hydrogen, H 2, in the reactivity series of metals - Example: CuO, PbO, FeO 10 Molecular Formula - The molecular formula of a compound gives us the actual number of atoms of each element that are present in a molecule of the compound. - Example: Compound Empirical formula Molecular Formula n Water H 2 O (H 2 O) 1 = H 2 O 1 Ethene CH 2 (CH 2 ) 2 = C 2 H 4 2 Glucose CH 2 O (CH 2 O) 6 = C 6 H 12 O 6 6

6 11 Ionic Formula - Ionic compounds are made up of positively-charged ions (cations) and negatively-charged ions (anions). Cation Formula Anion Formula Sodium ion Chloride ion Potassium ion Bromide ion Zinc ion Iodide ion Magnesium ion Oxide ion Calcium ion Hydroxide ion Iron (II) ion Sulphate ion Iron (III) ion Carbonate ion Copper (II) ion Nitrate ion Ammonium ion Phosphate ion Name of copound: Zinc bromide Name of compound: Aluminium oxide Cation Anion Cation Anion Zinc ion, Zn 2+ Bromide ion, Br - Aluminium ion, Al 3+ Oxide ion, O Formula of compound: ZnBr 2 Formula of compound: Al 2 O 3 12 Chemical equation Chemical equation is a shorthand description of a chemical reaction. A chemical equation is a representation of a chemical reaction in words or using chemical formulae. Qualitatively, a chemical equation shows: - types of reactants - types of products - the physical states of reactants and products Quantitatively, a chemical equation shows: - the information on the number of moles of the reactants - the information on the number of moles of the products Example: CaCO 3 (s) + 2HCl(aq) CaCl 2 (aq) + H 2 O(l) + CO 2 (g) Symbol Information which can be obtained CaCO 3, HCl Calcium carbonate and hydrochloric acid are reactants CaCl 2, H 2 O, CO 2 Calcium chloride, water and carbon dioxide are the products Represents produces (s) Represents solid state (aq) Represents aqueous state (l) Represents liquid state (g) Represents gaseous state According to the above chemical equation: 1 mol of calcium carbonate, CaCO 3 reacts with 2 mol of hydrochloric acid, HCl, producing 1 mol of calcium chloride, CaCl 2, 1 mol of water, H 2 O and 1 mol of carbon dioxide, CO 2.

7 Steps to write a balanced chemical equation Magnesium reacts with dilute hydrochloric acid, HCl to produce magnesium chloride, MgCl 2 and hydrogen gas, H 2. write an equation to represent the reaction Magnesium + hydrochloric acid Magnesium chloride + hydrogen gas Examples: 1. Hydrogen gas + oxygen gas Water 2. Potassium iodide + Bromine gas Iodine gas + Potassium bromide 3. Copper (II) oxide + Aluminium Aluminium oxide + Copper 4. Calcium carbonate Calsium monoxide + carbon dioxide Numerical problems involving chemical equations 1. Copper (II) oxide, CuO reacts with aluminium. Calculate the mass of aluminium required to react completely with 12g of copper (II) oxide, CuO. [RAM: O, 16; Al, 27; Cu, 64] 3CuO(s) + 2Al(s) Al 2 O 3 (s) + 3Cu(s) 3 mol 2 mol 1 mol 3 mol 12 g? The number if moles of 12 g CuO 12g 12g = 0.15mol 1 1 (60 16) gmol 80gmol 3 mol CuO react with 2 mol Al, 0.15 mol CuO react with? mol Al 0.15 The number of mol of Al = x2 0. 1mol 3 The mass of aluminium required = 0.1 mol x 27 g mol -1 = 2.7g

8 2. A student heats 20g of calsium carbonate, CaCO 3 strongly. It decomposes to calsium oxide and carbon dioxide. a) If the CO 2 produced is collected at room conditions, what is the volume? CaCO 3 CaO + CO 2 1 mol 1 mol 20 g? dm 3 The number of moles of 20g CaCO 3 20g 20g = 0.2mol 1 1 ( [16]) gmol 100gmol 1 mol CaCO 3 1 mol CO mol CacO mol CO 2 The volume of CO 2 = 0.2 mol x 25 dm 3 mol -1 = 4.8 dm 3 b) Calculate the mass of calcium oxide, CaO produced. CaCO 3 CaO + CO 2 1 mol 1 mol 20g? g The number of moles of 20g CaCO 3 20g 20g = 0.2mol 1 1 ( [16]) gmol 100gmol 1 mol CaCO 3 1 mol CaO 0.2 mol CacO mol CaO The mass of CaO = 0.2 mol x [ ] g mol -1 = 11.2 g

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