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# formulas - Relative mass formula atomic mass and empirical...

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11/09/11 Relative mass formula, atomic mass, Relative mass formula, atomic mass, and empirical formula and empirical formula

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11/09/11 Relative formula mass, M Relative formula mass, M r r The relative formula mass of a compound is blatantly the relative atomic masses of all the  elements in the compound added together. E.g. water H 2 O: Therefore M r for water = 16 + (2x1) = 18 Work out M r for the following compounds: 1) HCl 2) NaOH 3) MgCl 2 4) H 2 SO 4 H=1, Cl=35 so M r  = 36 Na=23, O=16, H=1 so M r  = 40 Mg=24, Cl=35 so M r  = 24+(2x35) = 94 H=1, S=32, O=16 so M r  = (2x1)+32+(4x16) = 98 Relative atomic mass of O = 16 Relative atomic mass of H = 1
11/09/11 More examples More examples CaCO 3 40 + 12 + 3x16 100 HNO 3 1 + 14 + 3x16 2MgO 2 x (24 + 16) 80 3H 2 O 3 x ((2x1) + 16) 4NH 3 2KMnO 4 3C 2 H 5 OH 4Ca(OH) 2

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11/09/11 Relative atomic mass Relative atomic mass The mass of an isotopic element relative to Carbon- 12. Example: chlorine occurs in isotope forms Cl-35 (75.5%) and Cl-37 (24.5%) Relative atomic mass = ((75.5x35)+(24.5x37))/(75.5+24.5)=35.5 Try this: neon-20 (90.9%), neon-21 (0.3%), and neon-22 (8.8%)
Calculating percentage mass Calculating percentage mass If you can work out M r  then this bit is easy… Calculate the percentage mass of magnesium in magnesium oxide, MgO: A r  for magnesium = 24 Ar for oxygen = 16 M r  for magnesium oxide = 24 + 16 = 40 Therefore percentage mass = 24/40 x 100% = 60% Percentage mass (%) = Mass of element A r Relative formula mass M r x100% Calculate the percentage mass of the following: 1) Hydrogen in hydrochloric acid, HCl

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formulas - Relative mass formula atomic mass and empirical...

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