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# C05 the mole concept

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• 1. Chapter 5 Mole Concept LEARNING OUTCOMES     Define the mole Derive empirical and molecular formulae State Avogadro’s Law Apply the mole concept to ionic and molecular equations
• 2. Chapter 5 Mole Concept Introducing the Mole A mole is the number of atoms or molecules in 1 g of hydrogen or 12 g of carbon. The number 6 x 1023 is called one mole or Avogadro’s constant in honour of Amedeo Avogadro. 1 mole of atoms of any element will have a mass equal to its relative atomic mass, expressed in grams.
• 3. Chapter 5 Mole Concept Formulas 1. Mass of 1 mole of atoms = Ar in grams Mass of the element in grams 2. Number of moles of atoms = Relative atomic mass, Ar 3. Mass of 1 mole of molecules = Mr in grams Mass of the substance in grams 4. Number of moles of molecules = Relative molecular mass, Mr 5. Mass of substance containing 1 mole of particles = Molar mass Actual mass of product obtained 6. Percentage yield = Theoretical mass of product obtainable
• 4. Chapter 5 Mole Concept Empirical Formula The empirical formula is the simplest formula.  It shows the simplest ratio of the elements present in a compound.  Examples of empirical formulae are: H2O, CO2, H2SO4, CH2, CH3   The following are not empirical formulae: (a) C2H4, (b) C2H6, (c) C2H4O2 because they can be reduced to: (a) CH2, (b) CH3, (c) CH2O
• 5. Chapter 5 Mole Concept Molecular Formula      The molecular formula is the true formula. It shows all the atoms present in the molecule. Examples of molecular formulae are: H2O, H2O2, CO2, H2SO4, Cu(NO3)2 Note that H2O is water, and the molecular formula is the same as the empirical formula. H2O2 is hydrogen peroxide. Its empirical formula is HO.
• 6. Chapter 5 Mole Concept Finding the Molecular Formula Worked example 1 Propene has the empirical formula CH2. The relative molecular mass of propene is 42. Find the molecular formula of propene. Solution   Let the molecular formula of propene be (CH2)n. Since the Mr is 42, (12 + 1x2)n = 42 14n = 42 n = 42 = 3 14 Hence the molecular formula is 3x(CH2) = C3H6
• 7. Chapter 5 Mole Concept Finding the Molecular Formula Worked example 2 A hydrocarbon consists of 85.7% of carbon and 14.3% of hydrogen by mass. (a) Find the empirical formula of the compound. (b) If the molecular mass is 56, find the molecular formula. Solution C : H (a) 85.7 : 14.3 85.7 : 14.3 12 1 7.14 : 14.3 7.14 7.14 1 : 2 The empirical formula is CH2. (b) Let the molecular formula be (CH2)n. Since the Mr is 56, (12 + 1x2)n = 56 14n = 56 n = 56 = 4 14 Hence the molecular formula is 4 x (CH2) = C4H8
• 8. Chapter 5 Mole Concept Formula of a compound A pure compound has a fixed chemical composition; hence it can be represented by a chemical formula.  For example, a molecule of water is made up of 2 atoms of hydrogen and 1 atom of oxygen, and its molecular formula is H2O.   We can find the formula of a compound from its percentage composition.
• 9. Chapter 5 Mole Concept Finding the formula of a compound Worked example 1 A compound of sodium contains the following percentage composition by mass: 32.4% of sodium, 22.6% of sulphur and 45.0% of oxygen. Find the formula of the compound. Solution Na : S : O Step 1: Write down the percentage: 32.4 : 22.6 : 45.0 Step 2: Divide each by the Ar: 32.4 : 22.6 : 45.0 (to get number of moles) 23 32 16 1.41 : 0.706 : 2.81 Step 3: Divide by the smallest number: 1.41 : 0.706 : 2.81 0.706 0.706 0.706 2 : 1 : 4 Step 4 : Write down the formula: Na2SO4
• 10. Chapter 5 Mole Concept Finding the formula of a compound Worked example 2 A compound contains 1.2 g of carbon, 3.2 g of oxygen and 0.2g of hydrogen. Find the formula of the compound. Solution Step 1: Write down the mass ratio: Step 2: Divide each mass by the Ar: Step 3: Divide by the smallest number: Step 4 : Write down the formula: C 1.2 1.2 12 0.1 0.1 0.1 1 : O : H : 3.2 : 0.2 : 3.2 : 0.2 16 1 : 0.2 : 0.2 : 0.2 : 0.2 0.1 0.1 : 2 : 2 CO 2H2 (or HCOOH)
• 11. Chapter 5 Mole Concept Finding the formula of a compound Worked example 3 A compound contains 48.6% of carbon, 43.2% of oxygen, with the remainder being hydrogen. Find the formula of the compound. Solution Step 1: Find the % of hydrogen: 100 – 48.6 – 43.2 = 8.2 % Step 2: Write down the % ratio: Step 3: Divide each mass by the Ar: Step 4: Divide by the smallest number: Step 5: Multiply each number by 2: Step 6 : Write down the formula: C : O : H 48.6 : 43.2 : 8.2 48.6 : 43.2 : 8.2 12 16 1 4.05 : 2.7 : 8.2 4.05 : 2.7 : 8.2 2.7 2.7 2.7 1.5 : 1 : 3 3 : 2 : 6 C3O2H6 (or C2H5COOH)
• 12. Chapter 5 Mole Concept Molar Volume of Gases Avogadro’s Law states that equal volume of gases under the same temperature and pressure contain the same number of molecules. Volume of 1 mole of gas = 24 dm3 Volume of gas = Number of moles x 24 dm 3 Number of moles = Volume of gas in dm3 24 dm3
• 13. Chapter 5 Mole Concept Limiting Reactants 2H2(g) + O2(g)   2H2O(g) 2 moles of hydrogen gas react with one or more moles of oxygen to form 2 moles of steam or water vapour. Therefore we say that oxygen is in excess and hydrogen is called the limiting reactant because the reaction stops when hydrogen is used up.
• 14. Chapter 5 Mole Concept Quick check 1 Find the formula of each of the following: 1. A compound containing 75% carbon and 25% hydrogen by mass. 2. A compound containing 46.7% silicon and 53.3% oxygen by mass. 3. A compound consisting of 43.4% sodium, 11.3% carbon and 45.3% oxygen by mass. 4. A compound consisting of 2.8 g of iron combined with 1.2 g of oxygen. 5. (a) A compound containing 18.9% lithium, 64.9% oxygen and the rest carbon by mass. (b) Give the name of this compound. Solution
• 15. Chapter 5 Mole Concept Quick check 2 1. A hydrocarbon consists of 80% carbon and 20% hydrogen by mass. (a) Find the empirical formula of the compound. (b) If the molecular mass is 30, find the molecular formula. 2. An acid contains 40% carbon, 6.67% hydrogen and 53.33% oxygen by mass. (a) Find the empirical formula of the acid. (b) If the molecular mass of the acid is 60, what is its molecular formula? 3. A compound called borazine has the following percentage composition by mass: 40.74% boron, 51.85% nitrogen and the rest hydrogen. Find the molecular formula of borazine, given it has a relative molecular mass of 81. Solution
• 16. Chapter 5 Mole Concept Solution to Quick check 1 1. CH4 2. SiO2 3. Na2CO3 4. Fe2O3 5. (a) Li2CO3 (b) lithium carbonate Return
• 17. Chapter 5 Mole Concept Solution to Quick check 2 1. (a) Empirical formula: CH3 (b) Molecular formula: C2H6 2. (a) Empirical formula: CH2O (b) Molecular formula: C2H4O2 (CH3COOH) 3. Molecular formula: B3N3H6 Return