Required Practical 1 Flashcards
Make up a volumetric solution and carry out a simple acid-base titration.
Describe how you would make up a volumetric solution.
METHOD
- Weigh the sample bottle containing the required mass of solid on a 2dp balance.
- Transfer to a beaker and reweigh sample bottle.
- Record the difference in mass.
- Add 100cm³ of distilled water to the beaker. Use a glass rod to stir to help dissolve the solid.
- Pour solution into a 250cm³ graduated flask via a funnel.
- Rinse the beaker and funnel and add washings from the beaker and glass rod to the volumetric flask.
- Make up to the mark with distilled water using a dropping pipette for last few drops.
- Invert flask several times to ensure uniform solution.
How do you reduce uncertainties in measuring mass?
- Use a balance that measures to more decimal places or using a larger mass will reduce % uncertainty in weighing a solid.
- Weighing sample before and after addition and then calculating difference will ensure a more accurate measurement of the mass added.
Describe how you would dilute a solution.
METHOD
- Pipette 25cm³ of original solution into a 250cm³ volumetric flask.
- Make up to the mark with distilled water using a dropping pipette for last few drops.
- Invert flask several times to ensure uniform solution.
What formula would you use to calculate the concentration of a diluted solution?
New diluted concentration = original concentration x (original volume ÷ new diluted volume)
Describe and explain a method for carrying out a titration.
METHOD
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Rinse equipment
- Burette with acid, pipette with alkali, conical flask with distilled water.
- Pipette 25cm³ of alkali into a conical flask and gently touch the end of the pipette onto the surface of the alkali to remove final drop from pipette.
- Add acid solution into burette and make sure jet space in the burette is filled with acid.
- Add a few drops of indicator.
- Use a white tile underneath the flask to help observe the colour change.
- Add acid to alkali whilst swirling the mixture and add acid dropwise at end point.
- Note burette reading before and after.
- Repeat titrations until at least 2 concordant results are obtained.
Why is a conical flask used as opposed to a beaker in a titration?
A conical flask is used in preference to a beaker because it is easier to swirl the mixture in a conical flask without spilling the contents.
Why do you have to ensure the jet space is filled properly prior to commencing the titration?
If the jet space is not filled properly prior to commencing the titration, it will lead to errors if it then fills during the titration leading to a larger-than-expected titre reading.
What are concordant results?
If 2 or 3 values are within 0.10cm³ and therefore concordant or close then we can say results are accurate and repeatable and the titration technique is good/consistent.
Why would you wash the sides of a conical flask during a titration? What could you use to do this?
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Distilled water can be added to the conical flask during the titration to wash the sides of the flask so that all the acid on the side is washed into the reaction mixture to react with alkali.
- It does not affect the titration reading as water does not react with the reagents or change the number of moles of acid added.
Describe how you would record results in a titration.
- Results should be clearly recorded in a table.
- Results should be recorded in full (i.e. both initial and final readings).
- Titres should be recorded to 2dp.
How would you calculate percentage uncertainty?
% uncertainty = uncertainity ÷ measuremant made on apparatus x 100
How do you reduce uncertainties in a titration?
- Replacing measuring cylinders with pipettes or burettes which have lower apparatus uncertainty will lower the % uncertainty.
- To reduce the % uncertainty in a burette reading it is necessary to make the titre a larger volume.
- This could be done by:
- **Increasing the volume and concentration of the substrate in the conical flask.
- Decreasing the concentration of the substance in the burette.
