C3 topic 2 revision: Quantitative analysis.

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Presentation transcript:

C3 topic 2 revision: Quantitative analysis. Water hardness. Molarity. Salts Titrations.

Water hardness Which ions make water hard? How can we test for water hardness? How can we remove temporary hardness? How can we remove permanent hardness? What do we need to do once all the Na+ is used up? Why don’t we use this method to remove temporary hardness?

Temporary hardness This can be removed by boiling. Caused by limescale (the product of magnesium and calcium ion decomposition) Removing the ions makes the water soft so it will lather.

Removing hardness.

Concentration

Converting You may also need to know that ... mole conc. (mol/dm3 ) x formula mass = mass concentration (g/dm3 ) mass conc. (g/dm3 ) / formula mass = mole concentration (mol/dm3)

Questions What is the mass concentration of sodium chloride (NaCl) in g/dm3 in a 1.50 mol/dm3 solution? At. masses: Na = 23, Cl = 35.5, formula mass NaCl = 23 + 35.5 = 58.5 mass conc. = mol conc. x formula mass Mass conc. In g/dm3 = 1.50 x 58.5 = 87.8 g/dm3

Calculate the concentration in (a) g/dm3 and (b) mol/dm3. A solution of calcium sulphate (CaSO4) contained 0.500g dissolved in 2.00 dm3 of water. Calculate the concentration in (a) g/dm3 and (b) mol/dm3. (a) concentration = 0.500/2.00 = 0.250 g/dm3 (b) At. masses: Ca = 40, S = 32, O = 64, f. mass CaSO4 = 40 + 32 + (4 x 16) = 136 moles CaSO4 = 0.5 / 136 = 0.00368 mol concentration CaSO4 = 0.00368 / 2 = 0.00184 mol/dm3

Making soluble salts (1)

Method 2 Which indicator to use? Methyl orange: strong acid / weak alkali Phenolphthalein: weak acid / strong alkali We usually use methyl orange in titrations as the colour change from orange to pink is easier to see.

Concentration = no. of moles volume Learn this formula triangle! m c v number of moles concentration (in mol dm-3) volume (in dm3)

Titration calculations cont. In a titration, 20 cm3 of 2.0 mol dm-3 HCl reacted with 25 cm3 of NaOH. What was the concentration of the sodium hydroxide? In a titration, 20 cm3 of 0.2 mol dm-3 HCl reacted with 50 cm3 of NaOH. What was the concentration of the sodium hydroxide?

Titration calculations cont. In a titration, 20 cm3 of 1.0 mol dm-3 sulphuric acid, H2SO4, reacted with 25 cm3 of NaOH. What was the concentration of the sodium hydroxide? In a titration, 20 cm3 of 1.0 mol dm-3 nitric acid, HNO3, reacted with 25 cm3 of NaOH. What was the concentration of the sodium hydroxide?