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ADDITIONAL SCIENCE FORM 4CHAPTER 9: MOLE CONCEPT9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Relative atomic mass of an element is defined as the mass of one atom of the element compared to 1/12 the mass of one carbon-12 atom. Relative atomic = ____Mass of one atom of X____ mass of X 1/12 x Mass of one C-12 atom

9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Example 1:A chlorine, CI atom is 2.92 times as heavy as acarbon-12 atom. What is the relative atomic mass of chlorine?SolutionRelative atomic mass of CI = 12 x 2.92 = 35 9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS The value of the relative atomic mass of an element may be obtained from the Periodic Table.

Proton numberSymbol of elementRelative atomic mass9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Relative molecular mass of an element or a compound is defined as the mass of one molecule of the element or compound compared to 1/12 the mass of one carbon-12 atom. Relative molecular = _ Mass of one molecule of X___ mass of X 1/12 x Mass of one atom of C-12 9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Example 1:A carbon dioxide, CO2 molecule is 3.67 timesas heavy as a carbon-12. What is the relativemolecular mass of CO2?SolutionRelative molecular = 12 x 3.67 mass of CO2 = 44 9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS If the chemical formula of a molecule is known, its relative molecular mass can be calculated. For element Relative molecular = a x Relative atomic mass of X mass of Xa For compound Relative molecular = (a x Relative atomic mass of X) mass of XaYb + (b x Relative atomic mass of Y) 9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Example 1:Calculate the relative molecular mass of ozone, O3. SolutionRelative molecular = 3 x Relative atomic mass of O3 mass of O = 3 x 16 = 489.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Example 2:Calculate the relative molecular mass of carbon disulphide, CS2.SolutionRelative molecular = Relative atomic mass of C mass of CS2 + (2 x Relative atomic mass of S) = 12 + 2 (32) = 76 9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS The term relative molecular mass is not appropriate for ionic compounds. Instead, the term relative formula mass is used. The relative formula mass is calculated the same way as relative molecular mass.

9.1 RELATIVE ATOMIC MASS AND RELATIVE MOLECULAR MASS Example 1:Calculate the relative formula mass of sodiumoxide, Na2O.Solution Relative formula = (2 x Relative atomic mass of mass of Na2O Na) + Relative atomic mass mass of O = 2 x 23 + 16 = 629.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESThe counting unit for atoms, molecules and ions is the mole (symbol mol). One mole of any substance contains 6.02 x 1023 atoms, molecules or formula units (for ionic compounds). One mole of copper contains 6.02 x 1023 atoms, one mole of carbon dioxide contains 6.02 x 1023 molecules and one mole of sodium chloride contains 6.02 x 1023 formula units.

9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESThe number 6.02 x 1023 is known as the Avogadros number (NA). The mass of one mole of a substance is called its molar mass (gmol-1). The molar mass of an atomic substance is its relative atomic mass.The molar mass of a molecular substance is its relative molecular mass. The molar mass of an ionic substance is its relative formula mass. 9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESThe relationship between number of moles, mass and molar mass: Number of moles = Mass of atoms of atoms Molar mass Number of moles = Mass of molecules of molecules Molar mass 9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESExample 1: The mass of a coil of copper wire is 0.30 g.What is the number of moles of copper atoms in the copper wire? Molar mass of copper is 63.5 gmol-1.Solution Number of moles of Cu atoms = 0.30 63.5 = 0.0047 mol 9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESExample 2:A 0.18 moles of sample of an oxide of nitrogen (X) was found to weigh 5.4 g. What is the molar mass of X?SolutionMolar mass of X = 5.4 0.18 = 30 gmol-1

9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESThe relationship between the number of atoms or molecules, number of moles and Avogadros number: Number of atoms = Number of moles x Avogadros of atoms number Number of molecules = Number of moles x Avogadros of molecules number9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESExample 1:What is the number of copper, Cu atoms in 4.7 x10-3 mol of copper?SolutionNumber of Cu atoms = 4.7 x 10-3 x 6.02 x 1023 = 2.8 x 1021 atoms9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESExample 2:A sample of sulphur dioxide, SO2 gas weighs5.0 g. What is the number of SO2 molecules in the sample? [Relative atomic mass: S = 32;O = 16]SolutionNumber of moles of SO2 = 5 64 = 0.078 mol

9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESNumber of SO2 molecules= 0.078 x 6.02 x 1023 = 4.7x1022 molecules9.2 THE RELATIONSHIP BETWEEN MOLES AND THE NUMBER OF ATOMS OR MOLECULESMass (g)Number of moles (mol)Number of particlesMOLAR MASSAVOGADROS NUMBERMOLAR MASSAVOGADROS NUMBER9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONTo write a chemical equation, you must know the reactants, the products and their chemical formulae. The chemical formulae of the reactants are written on the left-hand side while the chemical formulae of the products are written on the right-hand side. The number of each atom both sides of the equation must be balanced. 9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONExample 1: Write a balanced chemical equation for thereaction between hydrochloric acid and sodiumhydroxide solution to form sodium chloride andwater.Solution HCI + NaOH NaCI + H2O

9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONExample 2: Write a balanced chemical equation for thereaction between copper (II) oxide and hydrochloric acid to form copper (II) chloride solution and water.SolutionCuO + 2HCI CuCI2 + H2O

9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONExample 2: Write a balanced chemical equation for thereaction between copper (II) oxide and hydrochloric acid to form copper (II) chloride solution and water.SolutionCuO + 2HCI CuCI2 + H2O

9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONExample 3:A chemist wants to prepare 10.0 g of sodium hydroxide (NaOH) from the reaction of sodium,Na with water, H2O. Calculate the mass of sodium that he has to use in the reaction. Hydrogen gas is liberated in the reaction.[Relative atomic mass of Na = 23; Relative formula mass of NaOH = 40]9.3 THE MOLE CONCEPT IN A CHEMICAL EQUATIONSolution2Na + 2H2O 2NaOH + H2 Number of moles of NaOH = 10 40 = 0.25 molNumber of moles = Number of moles of Na of NaOH = 0.25 molMass of Na = 0.25 x 23 = 5.8 g

9.4 APPRECIATION THE MOLE CONCEPTThe law of conservation of mass states that in chemical reactions, the total mass of the reactants equals to the total mass of the products. The mole concept explains the law of conservation of mass because the number of atoms is still conserved.