1.1.7 Titrations

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Titrations allow you to find out exactly how much acid is needed to neutralise a quantity of alkali.
You measure out some alkali using a pipette and put it in a flask, along with some indicator e.g.
phenolphthalein. First of all, do a rough titration to get an idea where the endpoint is (the point
where the alkali is exactly neutralised and the indicator changes colour). Add the acid to the
alkali using a burette- giving the flask a regular swirl.
Now do an accurate titration. Run the acid in to within 2cm3 of the end point, then add the acid
dropwise. If you don't notice exactly when the solution changed colour you've overshot and your
result won't be accurate. Record the amount of acid used to neutralise the alkali. Its best to
repeat this process a few times, making sure you get get the same answer each time.
Indicators show you when the reaction's just finished. Indicators change colour as if by magic.
In titrations, indicators that change colour quickly over a very small pH range are used so you
know exactly when the reaction has ended. The two main two indicators for acid/alkali reactions
- Methyl orange, which turns yellow to red when adding acid to alkali
- Phenolphthalein, which turns red to colourless when adding acid to alkali
Example: 25cm3 of 0.5M HCl was used to neutralise 35cm3 of NaOH solution. Calculate the
concentration of the sodium hydroxide solution in mol dm -3.
First write a balanced equation and decide what you know and what you need to know:
HCl + NaOH NaCl + H2O
25cm3 35cm3
0.5 M ?
Now work out how many moles of HCl you have:
Number of moles of HCl= concentration x volume/1000 = 0.5 x 25 /1000 = 0.0125 moles
From the equation you know 1 mole of HCl neutralises 1 mole of NaOH. So 0.0125 moles of
HCl must neutralise 0.0125 moles of NaOH.
Now work out the concentration of NaOH:
Concentration of NaOH = Moles x 1000 / volumes = 0.0125 x 1000 / 35 = 0.36 mol dm-3


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