Chem

CHEMISTRY IN CONTEXT

Laboratory Manual Answers

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Practical 1
1 a. b. c. Hydrated means chemically bonded to water or containing water. Anhydrous means containing no water or without water. Water of crystallisation is the water present in the crystals of certain compounds such as Epsom salts.

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The crystals of Epsom salts are almost colourless. On heating, steam is given off andsome of this condenses to a colourless liquid (water) on the cooler (upper) parts of the test tube. Eventually, a white powder is left in the test tube. Allow water to condense on the cooler (upper) parts of the test tube. Then add a little white anhydrous copper sulphate to the water. This will turn to blue hydrated copper sulphate showing that water has been produced. To ensure that all the waterof crystallisation has been driven off. = 2.38 g Mass of hydrated MgSO4 used Mass of water in this sample of hydrated MgSO4 = 1.22 g Mass of MgSO4 in this sample of hydrated MgSO4 = 1.16 g a. b. Relative formula mass of water (H2O) = (2 × 1) + 16 = 18 Relative formula mass of magnesium sulphate (MgSO4) = 24 + 32 + (4 × 16) = 120

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From my results: 1.22 g of H2O combine with1.16 g of anhydrous MgSO4 in Epsom salts ⇒ 0.068 moles of H2O combine with 0.0097 moles of anhydrous MgSO4 in Epsom salts ⇒ 7.0 moles of H2O combine with 1 mole of anhydrous MgSO4 in Epsom salts Formula for Epsom salts is MgSO4.7H2O.

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Practical 2
1 Average accurate titration: 25.5 cm3 of 0.10 mol dm–3 HCl ≡ 25 cm3 of the diluted approx. 1.0 mol dm–3 NaOH 25.5 × 0.10 moles HCl ≡ 25cm3 of diluted approx. 1.0 mol dm–3 NaOH 1000 1 mole of HCl reacts with 1 mole of NaOH, ∴ concentration of diluted NaOh = 25.5 1000 × 0.10 × mol dm–3 1000 25

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= 0.102 mol dm–3 ⇒ concentration of approx. 1.0 mol dm–3 NaOH = 1.02 mol dm–3 3 4 Mass of aspirin tablets taken = 1.62 g To ensure that all the acetylsalicylic acid in the aspirin tablets is completely hydrolysed by the sodiumhydroxide. Vigorous heating and boiling may cause liquid to spit out of the conical flask. To ensure that all the hydrolysed contents in the conical flask are transferred to the 250 cm3 standard flask. 25 cm3 of the hydrolysed solution ≡ 12.2 cm3 of 0.10 mol dm–3 HCl ⇒ 250 cm3 of the hydrolysed solution ≡ 122 cm3 of 0.10 mol dm–3 HCl a. Moles of NaOH added to the flask before hydrolysis = 25 × 1.02 1000

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= 2.55 × 10–2 b. c. 8 9 Moles of NaOH remaining after hydrolysis = 122 × 0.10 = 1.22 × 10–2 1000

Moles of NaOH used in the hydrolysis = 1.33 × 10–2

1.33 × 10–2 2 1.33 Mass of acetylsalicylic acid hydrolysed = × 10–2 × 180 2 Moles of acetylsalicylic acid hydrolysed = = 1.20 g ∴ % of acetylsalicylic acid in tablets = 1.20 × 100 = 74% 1.62

10 After diluting the hydrolysed mixture,part of the remainder of the tablets appears as an insoluble white solid on the bottom of the standard flask. This was probably the coating on the tablets which might be calcium carbonate (chalk), silicon dioxide (silica) or starch. 11 The packet claims that each tablet contains 300 mg (0.30 g) of aspirin. Assuming that by ‘aspirin’ the manufacturers mean 0.30 g of acetylsalicylic acid, thiscompares with 1.20 g of acetylsalicylic acid in 5 tablets (i.e. 0.24 g per tablet) according to the experiment.

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Practical 3
1 2 5 moles of Fe2+ react with 1 mole of MnO4–. The tablets in the 1.0 mol dm–3 should not be heated more than necessary because Fe2+(aq) ions might be oxidised to Fe3+(aq). The tablets are dissolved in sulphuric acid rather than water because the reaction of MnO4– withFe2+ requires H+ ions. In fact, eight H+ ions are required for each MnO4– ion which reacts. The outer coating remains as a white insoluble solid. This might be calcium carbonate (chalk), silicon dioxide (silica) or starch. 25 cm3 of the Fe2+ solution reacted with 12.20 cm3 of 0.010 mol dm–3 MnO4–. This is 6 12.20 × 0.01 = 1.22 × 10–4 moles of MnO4– 1000

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∴ 25 cm3 of the Fe2+...