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IC5.3.4 Percentage masses from formulae © Oxford University Press Percentage masses from formulae
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IC5.3.4 Percentage masses from formulae © Oxford University Press The formula of a compound, and the relative atomic masses of the elements it contains, can be used to calculate percentage masses.
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IC5.3.4 Percentage masses from formulae © Oxford University Press Ammonium nitrate, NH 4 NO 3, is used as an artificial fertiliser. Suppose you wanted to calculate the percentage by mass of nitrogen in ammonium nitrate. Follow these three steps to see how it’s done…
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IC5.3.4 Percentage masses from formulae © Oxford University Press NH 4 NO 3 A r of N = 14, A r of H = 1, A r of O = 16 1.Calculate the relative formula mass of the compound using the relative atomic masses. M r of NH 4 NO 3 = 14 + (4 × 1) + 14 + (3 × 16) = 80
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IC5.3.4 Percentage masses from formulae © Oxford University Press NH 4 NO 3 A r of N = 14, A r of H = 1, A r of O = 16 2.Multiply the relative atomic mass of the element in the question by the number of its atoms in the compound. Mass of N = 2 × 14 = 28 (notice that N appears twice in the formula)
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IC5.3.4 Percentage masses from formulae © Oxford University Press NH 4 NO 3 A r of N = 14, A r of H = 1, A r of O = 16 3.Work out the percentage by mass of nitrogen in ammonium nitrate using the numbers from steps 1 and 2. Percentage of N in NH 4 NO 3 = 28/80 × 100% = 35%
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IC5.3.4 Percentage masses from formulae © Oxford University Press Now use these A r values to help you to answer the questions: H = 1, O = 16, Na = 23, S = 32, Cu = 63.5 1.Calculate the percentage by mass of sodium in sodium hydroxide, NaOH. M r of NaOH = 23 + 16 + 1 = 40 A r of Na = 23 Percentage of Na in NaOH = 23/40 × 100% = 57.5%
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IC5.3.4 Percentage masses from formulae © Oxford University Press A r values : H = 1, O = 16, Na = 23, S = 32, Cu = 63.5 2.Calculate the percentage by mass of oxygen in copper sulfate, CuSO 4. M r of CuSO 4 = 63.5 + 32 + (4 × 16) = 159.5 Mass of O = (4 × 16) = 64 Percentage of O in CuSO 4 = 64/159.5 × 100% = 40%
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