Required Practical 1 - Volumetric Solution + Titration Flashcards
Volumetric Solution
- Weigh known mass of solid using weighing bottle and record value
- Wash all into beaker using distilled water
- Dissolve in beaker by stirring with glass rod
- Wash out into a 250cm3 volumetric flask
- Add dropwise with teat pipette to marking/meniscus
- Fasten and invert flask several times
Titration
- Pipette a known volume of solution of an unknown concentration into a conical flask
- Add a few drops of phenolphthalein indicator
- Add volumetric solution to burette using a funnel
- Record start volume to 2 dp
- Titrate solution from burette until first permanent colour change
- Record rough titre to 2 dp
- Repeat until three results are recorded within 0.1cm3 to calculate a mean titre
Suggest a reason for having a rough titre
Allows a rough estimate to made for the titre so repeats can be titrated dropwise near that value to obtain an accurate result
Give a reason why rough titres are not included in calculating mean titre
Not within 0.1cm3 of other titres
Give hazards and safety precautions for acid-base titrations
- corrosive so wear eye protection/gloves
- irritant vapour so handle in fume cupboard
Suggest how you could reduce percentage uncertainty of titre and justify your choice
- More concentrated standard solution in conical flask
- Larger titre reading
Improvements for preparation of volumetric solution
- Wash out weighing bottle/beaker/stirring rod/funnel using distilled water (into volumetric flask)
- Use a teat pipette to make up mark on volumetric flask
- Invert flask
Purpose of titration
- determine concentration of a basic solution
- detect equivalence point for a acid-base reaction
Suggest why it is not important to fill burette to zero mark
Volumes are determined from difference between initial and final readings
Suggest significance of touching off hanging drop after closing burette
Liquid on tip of burette is part of volume delivered by burette
Suggest why you should not leave the funnel in the burette
- small drops of liquid might fall into the burette
- mean titre is lower than actual titre
Suggest why burette and pipette are rinsed with reagents they hold before titration
- ensure only appropriate reagent present
- prevent contamination or dilution of solution by water or other chemicals
- removed any air bubbles trapped in valve
Suggest why two or three drops of indicator are added to conical flask
- too little means colour is not visible
- too much could impact pH thus cannot determine true end point of reaction
Suggest why it is important to take burette reading at eye level or below meniscus
Avoid parallax error
Suggest why a conical flask is used in a titration rather than a beaker
Easier to swirl without losing liquid
Suggest why a burette is appropriate to use in an acid base titration
- can control amount of liquid released
- allows for extremely accurate addition of liquid
- precise readings can be taken
Purpose of volumetric pipette
Measuring and transferring an exact volume
Explain what is meant by concordant results
- within 0.1 od each other
- reproducible, consistent results
Suggest how to obtain an accurate average titre
- do further titrations (at least three)
- must be concordant
Suggest how you can accurately measure volume in burette
- take reading at eye level or below meniscus
- hold while tile behind burette so it is easier to read
Explain why using a burette increases the precision of the titration
- burette has high precision (scale measure in 0.1cm^3)
- precision is limited by scale of instrument used
Suggest why distilled water is used to wash conical flask near end point of titration
- returns reagent on sides of flask to reaction mixture
- ensure reagent is full reacted to give accurate end point
What effect will adding distilled water to conical flask near end point of titration have on titre
- no effect
- number of moles of solution does not change (only diluted)
- colour change determined by moles of solution
Suggest why concordant results are used to calculate mean titre
- within 0.1cm^3 of each other
- more reliable mean titre calculated
- reduced human error in titration
Suggest why conical flask should be rinsed with distilled water before titration
- holds certain number of moles of standard solution
- rinsing removes any residues that could add to the number of moles of solution
Suggest effect of bubble in burette on titre reading
- titre reading will be higher than actual titre
- when bubble bursts volume decreases but no reagent is added
How to calculate percentage error of equipment
Sum of individual percentage errors of equipment used
Describe steps taken during the titration procedure that help to ensure that accurate results are obtained
- conical flask rinsed with distilled water
- burette and pipette rinsed with reagents they hold
- funnel is removed once solution is transferred to burette
- only a few drops of indicator added to conical flask
- conical flask is swirled
- ensure burette has no air bubbles
- readings for burette are taken at eye level or below meniscus
- white tile held behind burette when readings taken
- distilled water used to wash sides of conical flask near end point
- solution is added dropwise near end point of titration
Suggest why it is important to fill the space below the burette tap with solution before beginning a titration
- space will fill during titration
- volumes added will be too large
- titre larger than true value
Suggest why it is important to carry our repeat titrations
- repeats improve reliability
- single titration could be anomalous
