Titration between HCl and K₂CO₃

Dr. Mudassar Altaf, Associate Professor of Chemistry

Paper: P3 Oct./Nov. 2022 9701/36

Question statement: In the experiment, you will determine the concentration of dilute HCl by further diluting it and then titrating with aqueous K₂CO₃. 

Solutions provided: 

• FB 1: dilute HCl solution.
• FB 2: 8.46g dm^-3 anhydrous K₂CO₃ solution.
• FB 3: Bromophenol blue indicator.

(a) Method:

• Use 10cm³ pipette to transfer 10.0cm³ of FB 1 into volumetric flask. 
• Make the solution up to 250 cm³ using distilled water.

Shake the volumetric flask and its contents thoroughly. Label this solution FB 4.

• Fill the burette with FB 4.
• Pipette 25.0 cm³ of FB 2 into conical flask.
• Add few drops of FB 3.
• Perform rough titration and record your burette readings in the space below
• The rough titre is ………………….. cm³. 

• Carry out as many accurate titrations as you think, necessary to obtain consistent results. 
• Make sure any recorded results show the precision of your practical work.
• Record in a suitable form below all your burette readings and the volume of FB 4 added in each accurate titration.

(b) From your accurate titration results, calculate a suitable mean value to be used in your calculations. Show clearly how you obtained this value.

25.0cm³ of FB 2 required ………………………. cm³ of FB 4. 

(c) Calculations:

i. Give your answers to c(ii), c(iv) & c(v) in appropriate number of figures.
ii. Use information on page 2 to calculate the amount in mol of K₂CO₃ present in 25.0cm³ of FB 2. Amount of K₂CO₃ = ……………. mol.

iii. Give the ionic equation for the reaction taking place in the titration in (a). Include state symbols. 

………CO₃^2-……..+………..H⁺………..→…………+…………

iv. Calculate the concentration in mol dm^-3 of HCl in FB 4.

v. Calculate the concentration in mol dm^-3 of HCl in FB 1.

(d) A student uses a solution containing 8.46g dm^-3 of hydrated K₂CO₃ solution in the titration of (a) instead of FB 2. State, whether the student’s titre would be larger or smaller than your titre. Explain your answer.

(e) A student carries out the titration described in (a). By mistake, a 25cm³ pipette is used to transfer 25.0cm³ (instead of 10.0cm³) of FB 1 into volumetric flask. The solution is made up to 250cm³ with distilled water and labelled as FB 4. State, how the subsequent experimental procedure could be modified so that the titre obtained in (b) is not altered by the mistake.