Physical 1 Section 1 - Unit 2: Required Practical - Titrations and Volumetric Solutions

Question 1

Suggest and explain one way to reduce the percentage uncertainty when using a burette (2 marks)

Answer 1

• Use a larger mass of solid OR use a more concentrated solution
• So a larger titre/reading will be needed

Question 2

In a titration experiment, what do you rinse the burette with at the start and why (2 marks)

Answer 2

• The acid
• If you rinsed it with something like distilled water, any droplets of water left over will dilute the concentration of the acid

Question 3

During titration, the inside of the conical flask is rinsed with distilled water. Suggest why rinsing improves the accuracy of the titre (2 marks)

Answer 3

• It ensures all reagents are able to react
• This will not affect the titration reading as water does not react with the reagents/change the number of moles of acid added

Question 4

Explain how a standard solution is made (8 marks)

Answer 4

• Weigh the sample bottle containing the solid on a (2 d.p.) balance
• Transfer solid to beaker
• Reweigh empty sample bottle
• Record the difference in mass
• Add 100cm3 distilled water and stir with a glass rod until all the solid has dissolved
• Pour solution into a 250cm3 flask via a funnel
• Rinse beaker and funnel and add washings from the beaker and glass rod to the flask
• Make up to the 250cm3 mark with distilled water
• Invert flask

Question 5

Explain two ways to reduce the uncertainty in measuring mass (2 marks)

Answer 5

• Using a more accurate balance or a larger mass will reduce the uncertainty in weighing a solid
• Weighing sample before and after addition and then calculating difference (weighing by difference) will ensure a more accurate measurement of the mass added

Question 6

Explain how you would carry out a titration experiment (6 marks)

Answer 6

• Rinse the equipment
• Pipette 25 cm3 of the alkali into a conical flask
• Touch surface of alkali with the pipette to ensure the correct amount is added
• Using a funnel, fill the burette with the standard solution of
  known concentration (make sure that the jet space is filled)
• Add a few drops of indicator: phenolphthalein [pink (alkali) to colourless (acid)] or methyl orange [yellow (alkali) to red (acid)]
• Use a white tile underneath the flask to help observe the colour change
• Add acid to the alkali whilst swirling the mixture and add acid dropwise at end point
• Note the burette reading before and after addition of acid
• Repeat titration until at least 2 concordant results are obtained (two readings within 0.1 of each other)

Question 7

Explain why the jet space in a burette must be filled before a titration (1 mark)

Answer 7

• If it fills during the titration it will lead to a larger than expected titre reading

Question 8

In a titration experiment, what do you rinse the pipette with at the start?

Answer 8

The alkali that’s going in - if you rinsed it with something like distilled water, any droplets of water left over will dilute the concentration of the alkali

Question 9

In a titration experiment, what do you rinse the conical flask with at the start?

Answer 9

The distilled water that’s going in - this won’t affect the moles of alkali being added

Question 10

What does it mean if solution A is titrated against solution B?

Answer 10

Solution A is in the conical flask and solution B is in the burette

Question 11

What are the potential errors in using a gas syringe (2)

Answer 11

• Gas escapes before the bung is inserted

- Some gases like CO2 or SO2 are soluble in water so the true amount of gas is not measured

Question 12

Explain why graduated flasks should not be heated (2 marks)

Answer 12

• Graduated flasks have a fill line on their neck to get an accurate volume
• Heating would cause the flask to expand and the volume would be inaccurate

Question 13

Explain how you would carry out a calorimetric experiment (7 marks)

Answer 13

• Wash the equipment with the solutions to be used and dry the cup after washing
• Put the polystyrene cup in a glass beaker for insulation and support
• Measure out the volume of solution with a pipette and transfer to the insulated cup
• Clamp a thermometer into place and immerse it into the solution
• Measure the temperature of the solution every 2-3mins
• At 3 mins, transfer the second reagent to the cup
• Stir the mixture and record the temperature every minute after addition for several minutes

Question 14

Explain how you would investigate how the rate of a reaction changes with temperature (6 marks)

Answer 14

• Measure 10 cm3 of HCl and 10 cm3 of sodium thiosulfate in separate measuring cylinders and put the solutions in separate boiling tubes
• Using a water bath, get the two solutions to a specific temperature by placing the boiling tubes in the water bath
• Place the flask on the centre of the large cross and add the sodium thiosulfate
• Then add the
  hydrochloric acid, start the stopwatch and swirl to mix the solutions.
• Stop the clock when the cross disappears and note the time
• Repeat the experiment for four different temperatures

Question 15

Suggest why burettes are more suitable to use than pipettes when adding a solution during titration (1 mark)

Answer 15

• It can deliver variable volumes