C2 Topic Two - Quantitative Analysis

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Measuring Amounts

'The Mole' is just a name given to the number 6.023 x 10^23 (like a million, billion etc)

Why that number? Because one mole of atoms/molecules of any substance will have a mass (in grams) equal to the relative formula mass (Ar or Mr) for the substance.

Carbon Ar - 12 - one mole = 12g CO2 Mr - 44 - one mole = 44g

Molar mass = the mass of one mole. So carbons molar mass is 12g.

mass in g of thing = number of moles x Mr of thing (element or compound)

number of moles = mass in g of thing / Mr of thing (element or compound)

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Find out how much substance is dissolved in solution by evaporating the water:

Take a sample of the solution e.g. 10g of 200g.

Weigh a clean, dry evaporating basin. Put 10g in basin.

Gently heat the basin to evaporate the water until you think all the water is gone. Weigh basin and remaining solid again. Reheat and reweigh the basin and contents until theres no further change in its mass - now all the water has evaporated.

Mass of basin and solid - mass of basin = mass of substance dissolved in 10g of solution. Then times back up to whole:10g (sample) x 20 = 200g, N x 20 = amount of substance

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Concentration - how much stuff is dissolved in a certain amount of solution                                                  measured in g/dm^3 (mass-concentration) or mol/dm^3 (mole-concentration)

to convert cm^3 to dm^3, dive by 1000 e.g. 6000cm^3 = 6dm^3

mass-concentration = mass (g) / volume (dm^3) - so if the volume is given in cm^3 you have                                                                                          to convert it

however, to convert between mass-concentration and mole concentration..

mole-concentration (mol/dm^3) = mass-concentration (g/dm^3) / Mr

mass-concentration (g/dm^3) = mole-concentration (mol/dm^3) x Mr

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Hard Water

Hard water wont easily lather with soap, instead it reacts with soap to make the precipitate scumThis is because of the calcium and/or magnesium ions in hard water. Hard water is caused when water flows over rocks/through soilds containing:

Magnesium sulphate MgSO4, which dissolves in water

Calcium Sulphate CaSO4 which kind of dissolves in water - causes permanent

Calcium carbonate which is found as chalk, limestone or marble. But when reacted with acid rain it become calcium hydrogencarbonate Ca(HCO3)2 which is soluble and dissolves in water, releasing more calcium ions. - causes temporary

Magnesium sulphate, calcium hydrogencarbonate and the resultant calcium is what makes tap water in some areas, hard. Can be softed by removing the calcium + magnesium ions.

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Removing hardness

Temporary caused by calcium hydrogencarbonate Ca(HCO3)2

Permanent caused by dissolved calcium sulfate and others

Temporary hardness can be removed by boiling as it decomposes the calcium hydrogencarbonate to form insoluble calcium carbonate - limescale on kettles. This doesnt work for permanent as heating sulfate ions does do anything.

Both hardness types can be removed using an ion exchange resin (=solid insoluble in water polymer). The resin contains sodium or hydrogen ions and exchanges them for the calcium/magnesium ions if you run a water supply through the resin.

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An acid-base titration is a neutralisation reaction where hydrogen ions from acid react with hydroxide ions in a soluble base.

H+ (aq) + OH- (aq) = H2) (l) - the products of acid-base titration always salt + water

Titrations let you find out exactly how much acid is need to neutralise a quantity of base etc

- Using a pipette + pipette filler, add '25cm^3' alkali to conical flask, and 2/3 drops indicator. Indicator used depends on strength of acid/base:                                                                  Phenolphtalein - weak acid strong alkali    Methyl orange - weak alkali strong acid             If both are strong, use any indicator

- Fill burette with acid. Below eye level - bad if looking up if you spill some whilst pouring

- Using burette, add acid to alkali drop by drop, regularly swirling flask. Go really slowly near end point - indicator becomes colourless when all alkali has been neutralised

- Carry out a rough titration then repeat at least 2 more. Get slower when near the right volume, so can find out exactly. When you get similar results each time = reliable.

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Titrations cont.

         number of moles

concentration   x   volume

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Preparing soluble salts

Making soluble salt using acid and insoluble reactant using metal carbonates/metal oxides

           acid          + insoluble base = soluble salt                                                          hydrochloric acid  + copper oxide   = copper chloride

Add the base to the acid, the solid base will dissolve in acid as it reacts. When the acid has been neutralised, the excess solid will sink to bottom. Then filter out excess solid to get a solution containing water and salt only. Heat gently to evaporate water and crystalize salt.

Making soluble salts using an acid and soluble reactant using hydroxides

However, you cant tell if the reaction is finished. You cant add excess alkali either and filter out whats left as the salt is soluble. You have to add exactly the righ amount of alkali to neutralise the acid - so do a titration tow work out the amount of alkali needed.

Then do reaction with right proportions. Dont need indicator - salt wont be contaminated. End solution is salt and water. Evaporate off water and left with pure,dry salt.

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