Titration Experiments
Amount of Substance Worksheet
Experiment 1
1. A student uses a titration to determine the concentration of a sulfuric acid. The student starts by preparing a solution of sodium carbonate with a known concentration. The student weighs out exactly 5.00g of sodium carbonate into a beaker. Then, enough distilled water was added to dissolve the solid. The solution was transferred to a volumetric flask and the solution was topped up with water until it almost reached the mark. The solution was inverted several times to mix and a dropping pipette was used to add the remaining volume of water up to the level stated on the volumetric flask.
a) Describe the technique that allows the student to be certain of the exact amount of solid sodium carbonate in the beaker.
1. Place a weighing boat on a balance.
2. Add 5.00g of solid to the weighing boat and make a note of the weight of the boat and the sodium carbonate.
3. Transfer the sodium carbonate to a beaker and then reweigh the weighing boat.
4. The difference in weight between the boat and the sodium carbonate, and the boat after transferring the sodium carbonate tells you the mass of sodium carbonate in the beaker.
2. Add 5.00g of solid to the weighing boat and make a note of the weight of the boat and the sodium carbonate.
3. Transfer the sodium carbonate to a beaker and then reweigh the weighing boat.
4. The difference in weight between the boat and the sodium carbonate, and the boat after transferring the sodium carbonate tells you the mass of sodium carbonate in the beaker.
b) State any alterations that are necessary to improve the method. Explain why they are important.
– The solution should be stirred to fully dissolve the solid.
– The beaker, rod and funnel should be rinsed and the rinsings should be poured into the flask. This prevents loss of sodium carbonate.
– Mix one final time to ensure that the concentration is even throughout.
– The beaker, rod and funnel should be rinsed and the rinsings should be poured into the flask. This prevents loss of sodium carbonate.
– Mix one final time to ensure that the concentration is even throughout.
c) Calculate the percentage uncertainty in the student’s method. Assume that the volumetric flask has an uncertainty of 0.1cm3.
Balance Error (2 readings)
(0.01 / 5) x 100 = 0.2%
Total Balance = 0.2 x 2 = 0.4%
Volumetric Flask Error (0.1 / 250) x 100 = 0.04%
Total Uncertainty 0.4% + 0.04% = 0.44%
Total Balance = 0.2 x 2 = 0.4%
Volumetric Flask Error (0.1 / 250) x 100 = 0.04%
Total Uncertainty 0.4% + 0.04% = 0.44%
2. The standard solution produced in question 1 was titrated against the sulfuric acid of unknown concentration. The burette was washed, and the sodium carbonate solution was poured into the burette up to the 0 line. A conical flask was washed, and a pipette (uncertainty of 0.1cm3) was used to transfer 25.0cm3 of sulfuric acid into the flask. A rough titration was carried out using phenolphthalein as an indicator. Then, the titration was repeated accurately three times, and an average was found. The results are as follows:
| Rough | 1 | 2 | 3 | |
|---|---|---|---|---|
| Start (cm3) | 0.0 | 23.5 | 45.7 | 68.2 |
| End (cm3) | 23.5 | 45.7 | 68.2 | 90.5 |
| Titre (cm3) | 23.5 | 22.2 | 22.5 | 22.3 |
a) What was the burette washed with?
Sodium carbonate – it has to be the same as the solution in the burette or it will dilute it.
b) What was the conical flask washed with?
Distilled water – you don’t want to change the number of moles in the flask.
c) Why was the experiment repeated three times?
In order to get two concordant results.
d) The inside of the conical flask was washed with distilled water during the titration. Why was this done?
To make sure that any solution that has splashed up and hit the inside of the flask has been washed into the solution.
e) What mistake was made during the titration?
Wrong indicator.
f) Why was it a mistake?
A strong acid (H2SO4) and weak base (Na2CO3) titration would require methyl orange, not phenolphthalein.
g) What is the concentration of the acid?
Mean Titre
Concordant results are 22.2 and 22.3.
Mean = (22.2 + 22.3) / 2 = 22.25 cm3
Mean = (22.2 + 22.3) / 2 = 22.25 cm3
Moles of Na2CO3
Mr = 106.0. Mass = 5.00g.
Total Moles in 250cm3 = 5.00 / 106.0 = 0.04717 mol
Concentration Na2CO3 = 0.04717 / 0.250 = 0.1887 mol dm-3
Total Moles in 250cm3 = 5.00 / 106.0 = 0.04717 mol
Concentration Na2CO3 = 0.04717 / 0.250 = 0.1887 mol dm-3
Stoichiometry
Na2CO3 + H2SO4 → Na2SO4 + H2O + CO2 (1:1 ratio)
Concentration of Acid
Moles Base used in Titre = 0.1887 x 0.02225 = 0.004199 mol
Moles Acid in 25cm3 = 0.004199 mol
Conc Acid = 0.004199 / 0.025 = 0.1679…
Answer = 0.168 mol dm-3
Moles Acid in 25cm3 = 0.004199 mol
Conc Acid = 0.004199 / 0.025 = 0.1679…
Answer = 0.168 mol dm-3
h) What is the percentage error of the titration?
Using typical uncertainty of ±0.15 cm3 per titre.
(0.15 / 22.25) x 100 = 0.67%
(0.15 / 22.25) x 100 = 0.67%
i) What is the combined percentage error of the whole experiment?
Pipette error: (0.1 / 25) x 100 = 0.4%
Burette error: 0.67%
Making the standard (balance + flask): 0.44%
Total: 0.4 + 0.67 + 0.44 = 1.51%
Burette error: 0.67%
Making the standard (balance + flask): 0.44%
Total: 0.4 + 0.67 + 0.44 = 1.51%
Experiment 2
1. Outline the steps required to create a 0.5moldm-3 solution of potassium hydroxide from solid potassium hydroxide. You have access to a 25cm3 pipette and a 250cm3 volumetric flask.
Answer
– Calculate the mass required for the solution.
– Place beaker on balance and add that mass of KOH.
– Add enough water to dissolve the KOH, stirring it with a stirring rod.
– Transfer the solution to a volumetric flask and rinse the beaker, rod and funnel, and transfer the rinsings to the flask.
– Top up the flask to just below the line and invert several times to mix.
– Add remaining water up to the line using a dropping pipette and invert several times to mix.
– Place beaker on balance and add that mass of KOH.
– Add enough water to dissolve the KOH, stirring it with a stirring rod.
– Transfer the solution to a volumetric flask and rinse the beaker, rod and funnel, and transfer the rinsings to the flask.
– Top up the flask to just below the line and invert several times to mix.
– Add remaining water up to the line using a dropping pipette and invert several times to mix.
2. Outline the steps required to complete a titration to determine the concentration of phosphoric acid using the potassium hydroxide solution obtained in question 1. In this titration, the solution in the burette is KOH.
Answer
– Rinse the burette with KOH and then fill up to the 0 mark with KOH.
– Rinse a conical flask with distilled water.
– Rinse a pipette with H3PO4 and transfer 25cm3 of H3PO4 to the conical flask. Add indicator.
– Add KOH to the H3PO4 until the indicator changes colour.
– Repeat the titration to a higher degree of accuracy (dropwise near end point).
– Repeat until concordant results.
– Rinse a conical flask with distilled water.
– Rinse a pipette with H3PO4 and transfer 25cm3 of H3PO4 to the conical flask. Add indicator.
– Add KOH to the H3PO4 until the indicator changes colour.
– Repeat the titration to a higher degree of accuracy (dropwise near end point).
– Repeat until concordant results.
3. During the titration, concordant results were obtained and the volume of 30.5cm3 was required to neutralise 25cm3 of phosphoric acid. Calculate the percentage error of the experiment.
Working Out
Balance Error
Moles KOH needed = 0.5 x 0.25 = 0.125 moles
Mass KOH = 0.125 x 56.1 = 7.0125g
Error = (0.01 x 2 x 100) / 7.0125 = 0.285%
Mass KOH = 0.125 x 56.1 = 7.0125g
Error = (0.01 x 2 x 100) / 7.0125 = 0.285%
Glassware Error
Volumetric flask: (0.3 / 250) x 100 = 0.12%
Pipette: (0.1 / 25) x 100 = 0.4%
Burette: (0.15 / 30.5) x 100 = 0.49%
Pipette: (0.1 / 25) x 100 = 0.4%
Burette: (0.15 / 30.5) x 100 = 0.49%
Total Error
0.285 + 0.12 + 0.4 + 0.49 = 1.295%