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3. Chemical Equations and Stoichiometry. 3.1 Formulae of Compounds 3.2 Derivation of Empirical Formulae 3.3 Derivation of Molecular Formulae 3.4 Chemical Equations 3.5 Calculations Based on Chemical Equations 3.6 Simple Titrations. Stoichiometry ( 化學計量學 )p.19.

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## Chemical Equations and Stoichiometry

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**3**Chemical Equations and Stoichiometry 3.1 Formulae of Compounds 3.2 Derivation of Empirical Formulae 3.3 Derivation of Molecular Formulae 3.4 Chemical Equations 3.5 Calculations Based on Chemical Equations 3.6 Simple Titrations**Stoichiometry (化學計量學)p.19**Deals with quantitative relationships (a) among atoms, molecules and ions RAM / RMM / Formula Mass**Stoichiometry (化學計量學)p.19**Deals with quantitative relationships (b) among the constituent elements of a compound Empirical / Molecular Formulae**Stoichiometry (化學計量學)p.19**Deals with quantitative relationships (c) among the substances participating in chemical reaction Calculations involving chemical equation**3.1**Formulae of Compounds**3.1 Formulae of compounds (SB p.43)**Empirical formula Shows the simplest whole number ratio of the atoms or ions present E.g. Methane, CH4 Sodium chloride, NaCl**3.1 Formulae of compounds (SB p.43)**Molecular formula Shows the actual number of each kind of atoms present in one molecule E.g. CH4 methane Ionic compounds do not have molecular formulae**3.1 Formulae of compounds (SB p.43)**E.g. CH4 methane Structural formula Shows the bonding order of atoms in one molecule**3.1 Formulae of compounds (SB p.44)**Different types of formulae of some compounds**3.2**Derivation of Empirical Formulae**3.2 Derivation of empirical formulae (SB p.45)**Example 1 Mg + N2 MgxNy 0.450 g excess 0.623 g From composition by mass Mass of N used = (0.623-0.450)g = 0.173 g**3.2 Derivation of empirical formulae (SB p.45)**Example 1 Mg + N2 MgxNy 0.450 g excess 0.623 g From composition by mass Mg3N2**3.2 Derivation of empirical formulae (SB p.45)**Example 3-3C Check Point 3-3A Water of Crystallization Derived from Composition by Mass Example 2 Q.14**CuSO4 : H2O =**Q.14 heat CuSO4.xH2O CuSO4 + xH2O 10.0 g 6.4 g (10.0–6.4) g = 3.6 g x = 5**CxHyOz**heat From combustion data Vitamin C CxHyOz + O2 CO2(g) + H2O(g) excess 0.2000 g 0.2998 g 0.0819 g**CxHyOz + O2 CO2(g) + H2O(g)**Mass of C in sample = mass of C in CO2 formed Mass of H in sample = mass of H in H2O formed Mass of O in sample = total mass of sample – mass of C – mass of H**C**H O Mass (g) 0.0818 0.0091 0.109 Number of moles (mol) Simplest whole no. ratio 3 4 3 C3H4O3**3.3**Derivation of Molecular Formulae**3.3 Derivation of molecular formulae (SB p.49)**What is molecular formulae? Molecular formula = (Empirical formula)n**3.3 Derivation of Molecular Formulae (SB p.49)**Example 3-3A Example 3-3B Molecular formula From empirical formula and known relative molecular mass Empirical formula Molecular mass**473 K, 1.00 atm**CnH2n+2(l) CnH2n+2(g) 3.28 dm3 12.0 g Q.15 = 142 g mol1 Relative molecular mass = 142**n = 10**Q.15 473 K, 1.00 atm CnH2n+2(l) CnH2n+2(g) 3.28 dm3 12.0 g RMM = 12n + 2n+2 = 142 The molecular formula is C10H22**C, H, O are present**Qualitative Analysis Structural formula CH3COOH Quantitative Analysis IR, NMR, MS Empirical formula CH2O Molecular formula C2H4O2 RMM = 60.0 Determination of Chemical Formulae Ethanoic acid**Na :**Al : S : Calculate the % by mass of the constituent elements of soda alum. Na2SO4·Al2(SO4)3·24H2O**H :**O : Calculate the % by mass of the constituent elements of soda alum. Na2SO4·Al2(SO4)3·24H2O**A certain compound was known to have a formula which could**be represented as [PdCxHyNz](ClO4)2. Analysis showed that the compound contained 30.15% carbon and 5.06% hydrogen. When converted to the corresponding thiocyanate, [PdCxHyNz](SCN)2, the analysis was 40.46% carbon and 5.94% hydrogen. Calculate the values of x, y and z. (Relative atomic masses : C = 12.0, H = 1.0, N = 14.0, O = 16.0, Cl = 35.5, S = 32.0, Pd = 106.0)**Let M be the formula mass of [PdCxHyNz](ClO4)2**Then, the formula mass of [PdCxHyNz](SCN)2 = M + 2(32.0+12.0+14.0) – 2(35.5+416.0) = M – 83.0 % by mass of C in [PdCxHyNz](ClO4)2 % by mass of C in [PdCxHyNz](SCN)2**Solving by simultaneous equations,**x = 14, M = 557 % by mass of H in [PdCxHyNz](ClO4)2 y = 28 557 = 106.0 + 12.014 + 1.028 + 14.0z + 2(35.5+416.0) z = 4**3.4**Chemical Equations**3.4 Chemical equations (SB p.53)**Chemical equations a A + b B c C + d D a, b, c, d are stoichiometric coefficients**3.5 Calculations Based on Equations (SB p.65)**Example 3-5A Example 3-5B Check Point 3-4 Calculations based on equations Calculations involving reacting masses**2.43 g**excess ? 3.4 Chemical equations (SB p.53) Example (a) Excess oxygen 2Mg(s) + O2(g) 2MgO(s)**3.4 Chemical equations (SB p.53)**Example (a) Excess oxygen 2Mg(s) + O2(g) 2MgO(s) 2.43 g excess ? = 4.03 g**3.4 Chemical equations (SB p.53)**Example(b) limiting reagent to be determined 2Mg(s) + O2(g) 2MgO(s) 2.43 g 1.28g ?**3.4 Chemical equations (SB p.53)**Example(b) limiting reagent to be determined 2Mg(s) + O2(g) 2MgO(s) 0.100 mol 0.040 mol ? Mg is in excess, O2 is the limiting reagent**Check Point 3-4**3.4 Chemical equations (SB p.53) Example(b) limiting reagent to be determined 2Mg(s) + O2(g) 2MgO(s) 0.040 mol 0.080 mol = 3.22 g Q.16, 17**P4 (s) + 5O2(g) 2P2O5(s)**Q.16 4.00 g 6.00 g**excess**limiting reactant P4(s) + 5O2(g) 2P2O5(s) 0.0323 mol 0.188 mol O2 is in excess**P4(s) + 5O2(g) 2P2O5(s)**Q.16 0.0323 mol 20.0323 mol = 9.17 g**Q.17**= 2 : 3 2Al(s) + 3Cu2+(aq) 2Al3+(aq) + 3Cu(s)**3.5 Calculations Based on Equations (SB p.66)**Calculations involving volumes of gases • Gases not at the same conditions • Gases at the same conditions • - Gay Lussac’s Law**CH4(g) + 2O2(g) CO2(g) + 2H2O(g)**2.8 dm3 25C 1.65 atm 35.0 dm3 31C 1.25 atm ? dm3 125C 2.50 atm O2 is in excess and CH4 is the limiting reactant**CH4(g) + 2O2(g) CO2(g) + 2H2O(g)**0.189 mol 0.189 mol ? dm3 125C 2.50 atm = 2.47 dm3**Gay Lussac’s law :**When gases reacts, they do so in volumes which bears a simple whole number ratio to one another, and to the volumes of gaseous products, all volumes being measured under the same conditions of temperature and pressure. Watch video**39.5 cm3**Fe as catalyst Ammonia nitrogen + hydrogen heat H2(g) + CuO Cu(s) + H2O(l) 20 cm3 10 cm3 29.5 cm3 2NH3(g) 1N2(g) + 3H2(g)**Gay Lussac’s law is an application of the Avogadro’s**law. a A(g) + b B(g) c C(g) + d D(g) At fixed T & P, n V**Gay Lussac’s law**a A(g) + b B(g) c C(g) + d D(g) At fixed T & P**CxHy(g) + O2(g) xCO2(g) + H2O(?)**Determination of Molecular Formula from Reacting Volumes of Gases For CxHy

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