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Titration to Standardise a Hydrochloric Acid Solution


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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Titration to Standardise a Hydrochloric Acid Solution
In this activity, a hydrochloric acid solution of unknown concentration is standardised against a solution of sodium carbonate of known concentration. This is done using the titration method. The sodium carbonate solution here is being used as a ‘standard solution’. A ‘standard solution’ is one whose concentration is known accurately and is stable.

The equation for the reaction is:
Na2CO3(aq) + 2HCl(aq) → 2NaCl(aq) + CO2(g) + H2O(l)

Method:
1. Rinse the burette with a little of the acid.
2. Fill the burette with the acid using the small funnel. Be sure to remove the funnel after filling.
3. Remove air bubbles from the jet of the burette by opening the tap to allow a little of the acid into a waste container.
4. Record the initial burette reading. Draw up a results table for the titration results.
5. Rinse the pipette with a small amount of the sodium carbonate standard solution.
6. Using the pipette filler, pipette 25.0 cm3 of the carbonate solution into a conical flask.
7. Add five drops of the methyl orange indicator to the solution in the flask. Note the colour of the solution.
8. Set up the apparatus, making sure that the tip of the burette is just inside the neck of the conical flask. Run in the acid from the burette. This should be done in increasingly smaller amounts.
9. Swirl – do not splash – the contents of the conical flask regularly during the addition of the acid.
10. Continue adding the acid until the colour of the solution suddenly changes from yellow to pink, ideally on the addition of just one drop.
11. Record the final burette reading and the volume of the acid used.
12. Repeat with fresh 25.0 cm3 portions of the carbonate solution until you obtain two values which agree within 0.10 cm3.
13. Record your data directly into your table as you progress through the experiment.
14. Use at least two consistent values to find the average (mean). Do not use the rough initial titration value.
15. Calculate accurately the concentration in mol/dm3 of the acid.
Watch the next video for step-by-step calculations for titration.



Microscale Method:
1. Rinse out your microburette (2 cm3 graduated pipette) with the standard hydrochloric acid solution.
a) Fill it up to the zero mark with the solution of hydrochloric acid. Make sure that there are no air bubbles in the disposable tip.
b) Place the microburette in the microburette stand as shown in the diagram.
c) Ensure that the disposable tip is not directly over the wells to be used.
2. Rinse out the propette (or micropipette) with the standard sodium hydroxide solution.
3. Add 1 cm3 of sodium hydroxide solution separately to wells F1, F2 and F3.
4. Add 1 drop of methyl orange indicator to each well.
5. Move the end of the microburette so that it is positioned over well F1.
6. a Carry out a ‘rough’ titration of the hydrochloric acid against the sodium hydroxide solution by adding 0.1 cm3 at a time.
b) After each addition, stir the mixture with a microspatula.
c) Continue adding the acid to the sodium hydroxide until the yellow colour just changes to orange. If it goes red, you have gone past the end-point.
d) Read the volume of solution from the microburette and record it in your results table. The volume added is called the ‘titre’.
e) If necessary, refill your microburette.
7. Carry out an accurate titration with the sodium hydroxide in wells F2 and F3.
a) Add the hydrochloric acid quickly to the sodium hydroxide solution until you get within 0.100 cm3 of your ‘rough’ titre.
b) Now add a drop at a time until the colour change to orange just occurs.
c) Note your result in the results table.
8. Repeat the titration until you get results which are within 0.010 cm3 of each other.

Use the average of the two titres which are within 0.010 cm3 of each other.
Volume of acid required = …………………… cm3
The concentration of the sodium hydroxide = 0.0500 mol/dm3
The equation for the reaction is: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
The equation shows that ……… mol of HCl react with 1 mol of sodium hydroxide.
The number of moles of sodium hydroxide = (0.05/1000) × 1 = 5 × 10−5 moles
Number of moles of acid = ……… moles
Concentration of acid = (number of moles acid/volume used in cm3) × 1000 = ………… mol/dm3
Carry out your calculations using your average titre.



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