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CHAPTER 3: CHEMICAL FORMULAE & EQUATIONS

CHAPTER 3: CHEMICAL FORMULAE & EQUATIONS. Chemical Formulae. 4S1 Tuesday, 1/3/11 10.45 – 11.55 am. Instead of writing long description of chemicals or substances, chemists write their chemical formulae. For example, hydrogen gas is written as H 2. So, what is a chemical formula ???.

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CHAPTER 3: CHEMICAL FORMULAE & EQUATIONS

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  1. CHAPTER 3: CHEMICAL FORMULAE & EQUATIONS Chemical Formulae 4S1 Tuesday, 1/3/11 10.45 – 11.55 am

  2. Instead of writing long description of chemicals or substances, chemists write their chemical formulae. For example, hydrogen gas is written as H2

  3. So, what is a chemical formula???

  4. Chemical formula = a representation of a chemical substance using letters for atoms & subscript numbers to show the numbers of each type of atoms that are present in the substance

  5. The letter ‘N’ shows the symbol of nitrogen atom N2 The subscript ‘2’ shows that there are 2 nitrogen atoms in a molecule of nitrogen gas

  6. There is no subscript here. This means that there is only 1 atom of oxygen in this molecule The letter show the symbols of the element Li2O The subscript ‘2’ shows that there are 2 lithium atoms in a molecule of lithium oxide, Li2O

  7. Empirical formula = the simplest whole number ratio of atoms of each element in the compound e.g.: formula of glucose = C6H12O6 E.F of glucose = CH2O

  8. Determining empirical formula of a compound

  9. Refer page 42 of Textbook E. F. = CaF2

  10. Refer page 42 of Textbook E. F. = Na2SO4

  11. Try Q3 of Work This Out 3.7 Calculation page 42

  12. Refer page 42 of Textbook E. F. = Al2S3

  13. If empirical formula is the simplest whole number ratio, what is molecular formula???

  14. Molecular formula = the actual number of atoms of each element that is present in a molecule

  15. Molecular formula = (Empirical formula)n

  16. Determining molecular formula of a compound Tips: must know RMM & E.F. of the cpd to know its M.F.

  17. Refer page 43 of Textbook E.F. = CH2, RMM = 42. M.F.? [RAM: H=1, C=12] Assume M.F. = (CH2)n Based on the M.F., RMM = n [12 + 2(1)] = 14n However, RMM = 42 14n = 42 n = 42 = 3 14 M.F. = (CH2)3 = C3H6

  18. Refer page 44 of Textbook E.F. = C2H5, RMM = 58. M.F.? [RAM: H=1, C=12] Assume M.F. = (C2H5)n Based on the M.F., RMM = n [2(12) + 5(1)] = 29n However, RMM = 58 29n = 58 n = 58 = 2 29 M.F. = (C2H5)2 = C4H10

  19. Try Q2 of Work This Out 3.8 Calculation page 44

  20. Refer page 44 of Textbook E.F. = CH2O, RMM = 60. M.F.? [RAM: H=1, C=12] Assume M.F. = (CH2O)n Based on the M.F., RMM = n [12 + 2(1) + 16] = 30n However, RMM = 60 30n = 60 n = 60 = 2 30 M.F. = (CH2O)2 = C2H4O2

  21. REMINDER!!! Memorise the ionic formulae of cations and anions for the next lesson

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