AISS 2024 Mole Notes

Chemistry 6092 1 Ahmad Ibrahim Secondary School Name:___________________________________ ( ) Class:________ Date:__________ Chapter 9 : The Mole A Relative Atomic Mass (Ar) Definition: The relative atomic mass (Ar) of an atom is the average mass of one atom of that element compared to 𝟏 𝟏𝟐 of the mass of one carbon-12 atom. For example: Chlorine consists of 2 isotopes  Cl-35 (75%) and Cl-37 (25%) Ar of each chlorine atom = (75 𝑥 35)+(25 𝑥 37) 100 = 35.5 Check Point 1 Calculate the Ar of the following elements, with the given isotopic abundances. Element Isotopes Relative Abundance Ar of Element (a) Carbon C-12 98.89% (98.9 𝑥 12)+(1.11 𝑥 13) 100 = 12.0111 = 12.0 C-13 1.11% (b) Neon Ne-20 90.5% (90.5 𝑥 20)+(0.3 𝑥 21)+(9.2 𝑥 22) 100 = 20.187 = 20.2 Ne-21 0.3% Ne-22 9.2% (c) Potassium K-39 93.38% (93.38 𝑥 39)+( 0.01 𝑥 40)+( 6.61 𝑥 41) 100 = 39.1323 = 39.1 K-40 0.01% K-41 6.61% At the end of this section, you should be able to:  Define relative atomic mass, Ar. Formula, Stoichiometry, Mole Concept  Relative Atomic Mass  Relative Molecular/ Formula Mass  Percentage Composition of an Element in a Compound  Mass of an Element in a given mass of a Compound

Chemistry 6092 2 From the Periodic Table, Check Point 2 Write down the relative atomic mass for the following elements: (a) Sodium, Na 23 (b) Calcium, Ca 40 (c) Oxygen, O 16 (d) Copper, Cu 64 B Relative Molecular/ Formula Mass (Mr) Definition: The relative molecular / formula mass of a compound is the average mass of one molecule of that element of compound compared to 𝟏 𝟏𝟐 of the mass of one carbon-12 atom. C12 6 Atomic (mass) number = Relative atomic mass Proton number At the end of this section, you should be able to:-  Define relative molecular mass, Mr  Calculate relative molecular mass (and relative formula mass) as the sum of relative atomic masses

Chemistry 6092 3 For example: Mr = (2 x 14) + (8 x 1) + (1 x 12) + (3 x 16) = 96.0 What is the relative molecular mass, Mr, of this compound? 2 8 1 3 How many atoms of each of these elements does it contain? Ammonium Carbonate (NH4)2CO3 Name the different elements present in this compound, and write down the relative atomic mass, Ar, of each element Element: N Ar: 14 Element:H Ar: 1 Element: C Ar: 12 Element: O Ar: 16

Chemistry 6092 4 Check Point 3 1 Find the relative molecular mass of each of the following compounds: (a) Carbon dioxide, CO2 (b) Ammonium chloride, NH4Cl (c) Ammonium phosphate, (NH4)3PO4 (d) MgSO4.7H2O 2 Calculate the value of x in CuSO4.xH2O, given the Mr of CuSO4.xH2O is 250. C Percentage Composition of an Element in a Compound To calculate the percentage composition of an element in a compound: Example 1: Find the percentage composition of copper in copper(II) sulfate CuSO4. 1 𝑥 64 64 + 32 + 4(16) 𝑥 100% = 40.0% Percentage composition of an element in a compound = 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒂𝒕𝒐𝒎𝒔 𝒐𝒇 𝒆𝒍𝒆𝒎𝒆𝒏𝒕 𝒙 𝑨𝒓 𝑴𝒓 𝒐𝒇 𝒄𝒐𝒎𝒑𝒐𝒖𝒏𝒅 𝟏𝟎𝟎% At the end of this section, you should be able to:-  Calculate % composition of an element in a compound when given appropriate information 12 + 16 + 16 = 44.0 14 + 4 + 35.5 = 53.5 (18 x 3) + 31 + 4(16) = 149 24 + 32 + 4(16) + 7(18) = 246 Mr of CuSO4.xH2O = 250 Mr of xH2O = 250 – 160 = 90 x = 90 18 = 5 percentage composition of = copper

Chemistry 6092 5 Example 2: Find the percentage composition of hydrogen in ethanoic acid, CH3COOH. 1 𝑥 4 12 + 3 + 12 + 16 + 16 + 1 𝑥 100% = 6.67% Example 3: Hydrated copper(II) sulfate crystals (CuSO4.5H2O) can be dehydrated by heating. Calculate the percentage composition by mass of water present in the molecule. 5 𝑥 18 160 + 5(18) 𝑥 100% = 36.0% D Mass of an Element in given mass of a Compound To calculate the mass of an element in a given mass of a compound: Example 1: Calculate the mass of Cu in 32 g of CuSO4. Mass of Cu = 64 160 𝑥 32 = 12.8𝑔 Example 2: Calculate the mass of water in 12.3 g of MgSO4.7H2O. Mass of water = 7 𝑥 18 24+32+4(16)+(7𝑥18) 𝑥12.3 = 6.3 Mass of an element in a given mass of a compound = 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒂𝒕𝒐𝒎𝒔 𝒐𝒇 𝒆𝒍𝒆𝒎𝒆𝒏𝒕 𝒙 𝑨𝒓 𝑴𝒓 𝒐𝒇 𝒄𝒐𝒎𝒑𝒐𝒖𝒏𝒅  𝒎𝒂𝒔𝒔 𝒐𝒇 𝒄𝒐𝒎𝒑𝒐𝒖𝒏𝒅 At the end of this section, you should be able to:-  Calculate mass of an element in a compound when given the mass of the compound percentage composition of = hydrogen percentage composition of = water

Chemistry 6092 6 Formula, Stoichiometry, Mole Concept Worksheet 9A Answer the following questions. 1 Calculate the Mr of the following substances. (a) Na2O 23 + 23 + 16 = 62.0 (b) H2SO4 2+ 32+ 4(16) = 98.0 2 Calculate the percentage composition of the following elements in the respective compounds. (a) Fe in FeCO3 56 116 𝑥 100% = 48.3% (b) O in NaNO3 3(16) 85 𝑥 100% = 56.5% (c) H in MgSO4.7H2O 14 246 𝑥 100% = 5.69% 3 A magnesium strip produces magnesium oxide when burnt in air. (a) Write the chemical formula of magnesium oxide. MgO (b) Calculate the percentage composition of magnesium in magnesium oxide. 24 40 𝑥 100% = 60.0%

Chemistry 6092 7 E The Mole - When we say: - A pair of oranges => 2 oranges - A dozen of eggs => 12 eggs - In Chemistry, when we say one mole = 6 x 1023 particles (This is also called the Avogadro’s constant.) - Particles can be atoms, molecules, compounds etc. - For example: 1 mole of carbon dioxide gas = 6 x 1023 carbon dioxide molecules 0.5 mole of carbon dioxide gas = 3 x 1023 carbon dioxide molecules Likewise, 1 mol of hydrogen gas = 6 x 1023 hydrogen molecules 2 mol of hydrogen gas = 1.2 x 1024 hydrogen molecules - What does it mean in a chemical equation? Example: 2 H2 + O2  2 H2O Instead of stating that 12 x 1023 hydrogen molecules react with 6 x 1023 oxygen molecules to produce 12 x 1023 water molecules, we can just simply state that 2 moles of hydrogen molecules react with 1 mole of oxygen molecule to produce 2 moles of water molecules. F The Molar Mass The molar mass of an element / molecule is the mass of one mole of atoms of the element / molecule. It is equal to its relative atomic mass (Ar) / relative molecular mass (Mr). At the end of this section, you should be able to:-  Measure the number of atoms, molecules or ions in moles  Use the Avogadro’s constant to calculate the number of moles and number of particles Formula, Stoichiometry, Mole Concept  The Mole  Calculate Empirical Formula of a Compound  Calculation of Molecular Formula  Molar mass, and its relationship with moles and mass  Molar volume, and its relationship with moles and volume At the end of this section, you should be able to:-  Define molar mass

Chemistry 6092 8 G Calculation of MOLE 𝒏𝒖𝒎𝒃𝒆𝒓 𝒐𝒇 𝒎𝒐𝒍𝒆𝒔 = 𝒎𝒂𝒔𝒔 (𝒈) 𝒎𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 ( 𝒈 𝒎𝒐𝒍) 1 Calculate the number of moles in (a) 64 g of oxygen gas Number of mole of oxygen = 64 16 𝑥 2 = 2.00 (b) 12.25 g of barium nitrate Number of mole of barium nitrate = 12.25 261 = 0.0469 2 Calculate the mass of (a) 3 moles of chlorine atoms Mass of chlorine atoms = 3 x 35.5 = 106.5 g (b) 0.2 mole of bromine gas Mass of bromine gas = 0.2 x (80 x 2) = 32 g (c) 0.6 moles of sulfuric acid Mass of sulfuric acid = 0.6 x 98 = 58.8 g

Chemistry 6092 9 H Calculate Empirical Formula of a Compound What is Empirical Formula? Sugar (glucose) has a molecular formula of C6H12O6. Its empirical formula is CH2O, which shows the ratio of the elements in the compound. Example 1: A compound consists of 7.0 g of nitrogen combined with 4.0 g of oxygen. Find the formula of the compound. N O Mass 7 4 No. of moles 𝒎𝒂𝒔𝒔 (𝒈) 𝒎𝒐𝒍𝒂𝒓 𝒎𝒂𝒔𝒔 ( 𝒈 𝒎𝒐𝒍) 𝟕 𝟏𝟒 = 𝟎. 𝟓