Finding the concentration of a solution of HCL

• Procedure

  1. Wash out the 250 cm3 volumetric flask with distilled water.

  2. Use the pipette to transfer 25.0 cm3 of the hydrochloric acid solution into the volumetric flask. Use distilled water to make the solution up to the mark.

  3. Prepare your apparatus for the titration. The burette should contain the sodium hydroxide solution (previously standardised) and the conical flask should contain the dilute hydrochloric acid solution.

  4. Pour a 25.0 cm3 aliquot of the diluted hydrochloric acid into the conical flask. Use phenolphthalein as the indicator.

  5. Titrate the contents of the flask against the sodium hydroxide solution. Record all burette readings to the nearest 0.05 cm3.

  6. The end point of this titration is indicated by the contents of the flask becoming pale pink. Continued swirling will cause the pink colour to fade and disappear. If the pink colour persists for 5 seconds or more, the end point has been reached.

  7. Conduct further titrations until you have two concordant titres.

  8. Ask your teacher or a technician to check one of your burette readings.

  9. Record your results in the table below.
• Analysis of results

  1. Calculate the mean titre using your two concordant results.

  2. Calculate the number of moles of sodium hydroxide that were contained in your mean titre.

  3. Calculate the number of moles of hydrochloric acid that were contained in the full 250 cm3 of diluted hydrochloric acid.

  4. Calculate the concentration of the original solution of hydrochloric acid.
• What is the objective?

  To find the concentration of a solution of hydrochloric acid
• What are the safety precautions ?

  Wear eye protection to prevent sodium hydroxide to enter

  avoid skin contact with the acid, alkali and indicator

  always use a pipette filler

  take care when clamping and filling the burette that it does not crack
• What are the important tips?

  Do not include your rough titration when calculating your mean

  give all burette readings to the nearest 0.05cm3

  when scaling to find the number of moles in the full volume of solution, use this equation:

  number of moles in full volume = full volume x number of moles in aliquot volume / aliquot volume in which you know the number of moles
• Write a chemical equation for the reaction of hydrochloric acid with sodium hydroxide

  HCL + NaOH -> NaCL + H2O
• Write an equation for the reaction of sodium hydroxide with carbon dioxide

  2NaOH + CO2 -> Na2CO3 + H2O
• What happens when sodium hydroxide solution is stored

  Reacts with carbon dioxide in the air
• Volume of sodium hydroxide solution required

  Does not change
• Explain why it is better to have a titre of around 25cm3 than of around 10cm3?


  The percentage error will be greater for smaller titres than for larger titres.
• The student forgot to rinse the burette with his solution of diluted hydrochloric acid

  What effect would this have on the concentration of the hydrochloric acid he calculates ?

  The acid in the burette will be diluted

  so the titre will be larger.

  This reduces the number of moles of acid calculated in the flask, so the

  calculation for the original concentration will be lower.
• Explain the effect on the calculated concentration of the acid if the student forced

  all of the hydrochloric acid out of the pipette, when transferring the original

  hydrochloric acid into the conical flask.

  The number of moles of acid in the flask increased

  so the diluted solution is slightly more concentrated, thus the titre is smaller.

  If the titre is smaller, then the apparent number of moles of acid in the flask

  will be more.

  Hence, the concentration calculated will be higher.