By doing a titration you are able to find out exactly how much acid is needed to neutralise a certain amount of alkali, exactly how much alkali is needed to neutralise a certain amount of acid.
Method
- Add 25cm3 of alkali to a conical flask (with a pipette and pipette filler)
- Add a few drops of phenolphthalein indicator
- Fill a burette with your acid (NOTE: ensure you have the burette below eye level incase the acid sprays out etc)
- Add the acid to the alkali a bit at a time (using a burette). NOTE: regularly swirl the conical flask to ensure the acid is evenly distributed throughout the alkaline
- Stop adding acid as soon as the solution in the conical flask changes colour (with phenolphthalein, it will go colourless) - this means that the alkali has been neutralised.
- Using the burette, record the amount of acid used to neutralise the alkali.
- Repeat a few times to avoid anomalies etc.
If you are asked to find out the amount in moles of acid needed to neutralise an alkali (or vice versa), just do the same as a mole calculation. For example...
It takes 30cm3 of sulphuric acid of an unknown concentration to neutralise 25cm3 of sodium hydroxide with a concentration of 0.1 moles per dm3. Find the concentration of the sulphuric acid.
- First, work out how many moles of the 'known' substance (sodium hydroxide) you have...
moles = concentration x volume
= 0.1 x (25/1000)
= 0.0025 moles
- Now, write down the equation to work out the ratio of acid : alkali...
2NaOH + H2SO4 ---> Na2SO4 + 2H2O
This means there are two moles of sodium hydroxide to every 1 mole of sulphuric acid.
- This means we need to half the amount of moles of sodium hydroxide to get the amount of moles of sulphuric acid...
0.0025/2 = 0.00125
- Now we know the amount of moles and volume of the acid, so we can work out the concentration...
concentration = moles / volume
= 0.00125 / (30/1000)
= 0.0417 moles per dm3
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