CPrac C - Titration

NOTE:

1. If 10.0 cm3 pipette is used, end-point volume should be about 10.00 cm3

2. Measuring cylinders and beakers cannot be used for dilution as they are not calibrated for precise volume measurements and they have wide openings and lack narrow necks, making it difficult to achieve precise dilution

3. The total quantity for each reagent used should not exceed the quantity provided

4. Choose a volume of solution to be placed in the conical flask (10, 25) 

5. Calculate the unknown quantity in the question (Eg. Titre value) 

Describe titration general planning

1. Weigh accurately the solid sample (state the mass used) and dissolve the entire solid sample in a beaker first before transferring quantitatively into the standard or volumetric flask

2. Top up to the mark with distilled water 

3. Shake the mixture well to obtain a homogenous solution.

4. Fill up burette with FA2

5. Pipette 10.0 (or 25.0) cm3 of FA1 into a conical flask. Add 2 to 3 drops of ___ indicator into the conical flask.

6. Titrate the mixture against FA2 from the burette until a distinct colour change is observed to obtain the end–point.

7. Repeat the titration as many times as necessary to obtain consistent results to within 0.10 cm3

Quantitively transfer means…

Rinse beaker, filter funnel and stirring rod with deionised water ensuring no solute is left in the beaker and transfer all washings into the volumetric flask 

1. Pipette has greater % error due to smaller capacity 

2. Titre volumes between 0.1cm^3! 

Dilution answering

• Pipette 10.0 cm3 (or 25.0 cm3) of the solution into a 100 cm3 (or 250 cm3) volumetric flask

• Make up to the mark with deionised water and shake well to obtain a homogeneous solution

Why a pipette and burette used instead of measuring cylinder 

• They are very precise apparatus for measuring volume of solutions -> reduce % of error 

Explain how alkaline impurities in acid affect the titration value

• The alkaline impurities will react with some of the acid in the conical flask , there are now less H+ ions present which requires less OH- ions to neutralise it. Hence , the volume of NaOH needed for neutralisation is lower than expected. 

Why must you ensure that all the solid is dissolved in the beaker before transferring the solution into the volumetric flask?

• If any solid remains in the beaker during the transferring of solution, there is a risk of accidentally adding excess water to wash the solid into the flask resulting in solution volume exceeding calibration mark 

Explain how the to conduct titration quickly to estimate titre-volume 

Replace pipette with teat pipette (dropper), add 10 drops of Fa1 into a test tube, followed by 1 drop of indicator. Replace the burette with a teat pipette (dropper), add FA2 into the test tube dropwise and count the number of drops of FA2 needed for colour change. The number of drops of FA2 should be similar to the number of drops of FA1, if the number of drops is too large, FA1 should be diluted. 

Why you should avoid using too many drops of indicator

Indicators are weak acids or bases, adding too many drops of indicator may affect the concentration of acid or base present in the conical flask. The colour will be more intense and less distinct making it harder to detect the exact movement of the reaction until it reaches completion.

Suggest and explain changes to be made that will enable the experiment to be carried out successfully

Dilute the NaOH solution by a factor of 10 so that the new concentration is 0.1 / 10 = 0.01 mol dm–3. This will ensure that the volume of HCl required to reach the end-point in the burette will be about 25.0 cm3, which is within the measurement range of a burette.

Give 1 reason why thymolphthalein is used 

• Working range of indicator lies on end point (NOT equivalence) at rapid pH change of rotation 

Suggest which experiment has the largest % uncertainty with titre value

The experiment with 20.00 cm3 of FA 3 used as it has the smallest titre value