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Presentation On Stoichiometry: Relative Masses Of Atoms And Molecules

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Published in: Chemistry | Science
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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. Relative Masses of Atoms and Molecules Chemistry (IGCSE / O-Level)
  2. What will you learn in this lesson? • Relative atomic masses of elements • Relative molecular or formula mass of compounds • Calculate reacting masses
  3. Relative Atomic Masses of Elements As masses of the atoms are very small. So, it is convenient to measure the mass relative to each other. The standard atom used for this purpose is carbon-12 isotope, the 'mass' of which is given exactly 12 atoms mass I unit each helium -12
  4. Relative Atomic Masses of Elements (contd.) Relative atomic masses of element that has isotopes can be found relative to their other isotopes (This topic is already covered previously) 82 11 19 83 117 118 119 37 120 Figure 4.17: many different elements have more than one isotope. These bars show the proportions of different isotopes for some elements. Fluorine is rare in having just one isotope.
  5. Relative Molecular (Formula) Mass of Compounds The mass of a substance is the sum of all relative atomic masses of the atoms present in a molecule. • Relative Molecular Mass: The mass of a simple covalent molecule substance is called relative molecular mass • Relative Formula Mass: If a substance is ionic or giant covalent compound such as (silicon(lV) oxide), then the mass will be called relative formula mass
  6. Relative Molecular (Formula) Mass of Compounds (contd.) nce Formula Relative atomic masses. hydrogen cScium cac03 (one Ca2+ ion, one c032- ion) mm-nonium (two NH' SOF ion) mos04.7H20 (one IS hydrated ion. one magnesium sulfate S042—ion, 14M seven H-20 70 mdecules) Table 4.6: Reative formula masses of some elements and compounds. mass, MP 12 2 x 16 3 x 16 14 32 4 X 16 24 4 X 16 14 x 1 7 x 16
  7. Relative Molecular (Formula) Mass of Compounds (contd.) C'alculate the relative molecular or formula masses of the following substances: a oxygen, 02 b ammonia, NH. c sUfuric acid, H2S04 d potassium bromide, KBr e copper (An: H = 1, N = 14.0=16, S 32, K -39, cu 80)
  8. Calculating Reacting Masses The proportions of elements present in a compound always remain same. For example, magnesium oxide formed by magnesium and oxygen will always contain 60% magnesium and 40% oxygen by mass If 0.24 g of magnesium with 0.16 g of oxygen to produce 0.40 g of magnesium oxide (Figure 4.19), how much magnesium oxide (Ngo) will be produced by burning 12 g of magnesium? We have: 0.24 g Mg prodüting 0.40 g MgO so 1 g Mg producesL-4Q g MgO — g moo so 12 g 12 * 1.67 moo — 20 g mgo
  9. Calculating Reacting Masses (contd.) using ms method based on Simølegroportions; calculate the massofca1Zium oxide if XO g of calcium carbonate •re decomposed completely by heaüng, A previous experiment had shown that2010 g of calcium carbonate produces 11.2 g •f calcium oxide When lheatedl Calcin carbonate has the formula CaC03. a Mratis its relative formula mas Cdr)? (At: ca - 40, C- 12, O - 16) b 100 g of calciun carbonatecontain 40 g ofcalcium, how much calcium is present in 15 g of calcium carbonate?