How much alkali neutralises the acid?
Hydrochloric acid and sodium hydroxide react the moment they meet, and nothing on the bench shows it: no colour, no gas, no solid. The only way to know whether the acid is used up is to count. This activity pours the alkali in at half the acid's strength, so the first pour falls short and the pH paper says so. The working says by how much, and the next pour finishes the job. Then it asks how concentrated the salt you made is.
What you will use
Apparatus
- Beaker
Chemicals
- Dilute Hydrochloric Acid
- Sodium Hydroxide
The steps
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Pour dilute hydrochloric acid into the beaker, and choose 1 M.
Ten millilitres of 1 M acid is 10 millimoles of HCl.
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Pour in sodium hydroxide, and choose 0.5 M: half the acid's strength.
Nothing to see. The reaction is over already. Read the working under the step.
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How many millimoles of sodium hydroxide went in?
The same volume as the acid at half the strength, so half the moles.
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Which reagent ran out first? Give its formula.
One NaOH for one HCl, so whichever there is fewer moles of runs out.
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Dip the pH paper. Is it neutral yet?
Still acidic. Half the acid had nothing left to react with.
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Pour another 10 mL of 0.5 M sodium hydroxide, so 20 mL in all.
Now the moles match: as much alkali as acid.
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Dip the pH paper again.
Neutral. Neither reagent is left over, so nothing is pulling the pH either way.
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What is the concentration of sodium chloride in the beaker, in M?
Moles of salt divided by the volume of the WHOLE mixture in litres: the acid and both pours of alkali.
What it shows
You could not see the acid being used up. You could only count it. Neutral is not a colour the reaction reaches by itself. It is the point where the moles of alkali equal the moles of acid, which is what every titration is looking for.