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Presentation On Stoichiometry: The Mole And Avogadro\'s Constant

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This PPT is suitable for the IGCSE and O Level Chemistry students. It only provides theoretical background of the topic. The practice questions, worksheets and topical questions will be discussed during the tutoring sessions

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  1. The Mole and Avogadro's Constant Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • The mole and Avogadro's Constant • Calculations involving mole and mass of substance • Empirical Formulae of giant structures • Empirical and Molecular Formulae of simple covalent molecules
  3. The mole —the chemical counting unit Chemists count atoms and molecules by weighing them. The standard unit of the amount of substance is taken as the relative molecular (or formula) mass of the substance in grams. The unit is called 1 mole (shorthand or symbol 'mol') of the substance The mass of 1 mol of substance is called molar mass of the substance
  4. The mole —the chemical counting unit (contd.) carbon iron hydrogen water magnesium Oxide calcium carbonate silicon(iv) oxide Formula c H2 02 H 20 caC03 Si02 mass, Mr 12 56 2 x 1=2 2 x 16 = 32 (2 x 1) 16 = 18 24 = 40 40 + 12 + (3 x 16) 100 28 + (2 x 16) = 60 Mass of mole (molar mass) 12 g 56 g 32 g 18 g 40 g 100 g 60 g This mass (1 mole) contains 6.02 x 1023 carbon atoms 6.02 x 1023 iron atoms 6.02 x 1023 H2 molecules 6.02 x 1023 02 molecules 6.02 x 1023 H20 molecules 6.02 x 1023 MgO 'formula units' 6.02 x 1023 cac03 'formula 6.02 x 1023 Si02 'formula units'
  5. Avogadro Constant The number (6 x 1023) of characteristic particles in 1 mole of a substance 6.02* 100 6.02* Ion 6.02* 1023 ö2xt atoms molecules Ions formula electrons units One mole always contains the same number of particles (Avogadro constant), divi& mogadro' m Wltrply by molar mass. Mr number Of rurnber Of moles, n by mogadro' divide by molar mass, Mr What are the number of particles and mass of 1 mole of substance (atoms, molecules or formula units)?
  6. Calculations involving the Mole grams number of moles molar mas grams per mole mass no. of molar
  7. Calculations involving the Mole (contd.) I n»les are there in SO g o f sodi um h yd m Nide? The re—eve forrnula mas sodium hydroxlde Is: = 23 16 + 1 = 40 mola r ma— of NaOH 40 g/rno' 60 g no. of 40 g/mof moles Figure 5.8: Calculation triangle for working out the number of moles in 60 g of sodium hydroxide. nunber of = — (Figure 5.8) Of 1.5
  8. Calculations involving the Mole (contd.) 1 0.25 mol of water has a mass of 4.5 g. What is the molar mass of water? 2 How many moles are there in 12.75 g of aluminium oxide? (Ar: H 1; O = 16; Al = 27) What is the mass of 0.5 mol of hydrated copper(ll) sulfate crystals?
  9. Empirical Formulae of Giant Structures With the help of mole we can now workout chemical formulae from the experimental data on combining masses When the elements are given in grams, nd the numbe d the numb of moles of Of grams of the atoms of each elements that element that combine. combine. Find the simplest whole number ratio.
  10. Empirical Formulae of Giant Structures (contd.) If 0.24 g of magnesium is burnt, then 0.40 g of magnesium oxide is formed. Find the empirical formula of magnesium oxide? mass cornblned molar mass number of moles simplest ratio Mg 024g 24 g/mol 0M mdl o 0.16g 16 g/mol 001 mol Magnesium and oxygen are combined in the ratio of 1:1 to form a giant ionic lattice of Mg2+ and 02' Mgo
  11. Empirical Formulae of Giant Structures (contd.) When the elements are given in percentage of mass, Percentage. find th mass of each element in Img Of compoupu. empirical formula Find the number Of moles of each element present Find the simplest whole number
  12. Empirical Formulae of Giant Structures (contd.) A sample of silicon(lV) oxide is found to contain 47% by mass of silicon. How can we find its empirical formulae? percentage by mass 47% mass in ICOg molar mass number of moles simplest ratio Formula 47 g 28 g/mol 1.68 mol o 100 - 47 - 53 g 16 g/mol 3.31 mol 2
  13. Empirical and Molecular Formulae of Simple Covalent Molecules Not all structures are giant structures. For covalently bonded lattice molecules, both empirical and molecular formula are needed. Molecular formula can be found using relative molecular mass The method is similar to previous methods of finding empirical formula but the relative molecular mass of the substance is used to compare with relative molecular mass of the empirical formula of that substance
  14. Empirical and Molecular Formulae of Simple Covalent Molecules (contd.) Phosphorus burns in air to produce white clouds of phosphorus oxide. From experiments, it is found that oxide contains 44% of phosphorus percentage by mass in 100 g molar mass number of simplest ratio Formula 44 g 31 g,'mol zoo - 44 = 56 9 16 *'moa p20 It is found experimentally the relative molecular mass is 284. So, comparing with the empirical formula mass of P205 i.e. 142. The molecular formula of the molecule will be (P205)2 or P4010
  15. Formula of Hydrated Salt The mass of water present in crystals of hydrated salts is always a fixed proportion of the total mass If 5.0 g of hydrated copper(ll) sulfate is heated. The dehydrated remaining solid has the mass of 3.2 g OS04 3.2 g 160 g/mol 0.02 mol Molar mass Number of moles Simplest ratio Formula 5.0- 3.2 = 1.8 g 18 g/mol 0.10 mol CuS04.5H20
  16. Practice Questions 1 2 3 a Calcut*e the number of moles of sodium hydroxide in 16.0 g of sodium hydroxide (NaOH). b Calculate how many formula units of sodium hydroxide are present in 16.0 g of NaOH. From your answer, deduce how marw sodium ions (Na+) and hydroxide ions (OH — ) are present in this mass of sodium One of the ores of copper is the mineral chalcopyrite. A laboratory analysis of a sample showed that 15.15 g of chalcopyrite had the following compostMon by mass: copper 5.27 g and tron 4.61 g, Sulfur ts the only other etementpresent. Use these figures to calcülate the empirical formula of chalcopyrite. (AF S = 32, 56, cu = 64) A sample of antifreeze has the composition by mass: 38.7% carbon, 9.7% hydrogen, 51.6% oxygen. a b c H 12, o 16) Cälcul*e its empirical frmula. The relative molec&r mass of the compound is 62. What is its molecular formula? This anvcnmd is dial. The molecule contains two alcohol (—OH) groups attached to different carbon atoms. What is its displayed formula?

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