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Percentage Mass and Formulae in Chemistry Problems

Learn how to calculate the percentage mass of elements in compounds and determine the formulas of compounds using their masses and relative atomic masses (RAMs).

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Percentage Mass and Formulae in Chemistry Problems

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  1. C2 3.3 Percentages and formulae What is the percentage mass of Hydrogen in methane; CH4 C=12, H=1 ? Adding RAMs 12 + (1x4) = 16 Mass of Element as a fraction of compound 4 16 X by 100% 4 x 100% 16 = 25%

  2. C2 3.3 Percentages and formulae What is the percentage mass of Nitrogen in the fertiliser, Ammonium nitrate; NH4NO3 H = 1, N = 14, O = 16 ? Adding RAMs 14 + ( 1 x4) + 14 + ( 16 x 3 ) = 80 Mass of Element as a fraction of compound 28 80 X by 100% 28 x 100% 80 = 35 %

  3. C2 3.3 Percentages and formulae What is the formula of aluminium chloride if 9g of aluminium reacts with 35.5g of chlorine ? (RAMs Al = 27g, Chlorine = 35.5g) Divide mass by RAM (1st. Element) For aluminium: 9 = 0.333’ 27 For chlorine: 35.5 = 1.00 35.5 Divide mass by RAM (2nd. Element) Divide by smallest to find ratio 0.333 = 1 0.333 1.000 = 3 0.333 The formula of aluminium chloride is AlCl3

  4. C2 3.3 Percentages and formulae What is the formula of a compound which contains 16g of sulfur and 24g of oxygen? (RAMs S = 32g, Oxygen = 16g) Divide mass by RAM (1st. Element) For sulfur: 16 = 0.5 32 For oxygen: 24 = 1.50 16 Divide mass by RAM (2nd. Element) Divide by smallest to find ratio 0.5 = 1 0.5 1.5 = 3 0.5 The formula of sulfur trioxide is SO3

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