practical

Question 1

why do we rinse the burette first with denise’s water and then with solution?

Answer 1

1. deionised water: helps to clean burette by removing unwanted chemicals from previous titrations
2. solution: remove any traces of water left + prevent dilution of subsequent solution added to fill the burette

Question 2

explain why a conical flask and not a beaker was used for titration

Answer 2

1. conical flask has sloping sides that allow swirling of solution without any spillage during titration
2. swirling a beaker of solution during titration is likely to lead to spillage, giving rise to inaccurate titration result

Question 3

why do we need to swirl the conical flask continuously during titration?

Answer 3

1. ensure reactants are mixed well to facilitate reaction
2. with no continuous swirling, reactants are only mixed in localised region for reaction
3. any colour change in localised region may be mistaken for end-point being reached
4. give rise to inaccurate titration results

Question 4

why is there a need to rinse inner walls of conical flask with deionised water during titration?

Answer 4

1. helps to flush down any untreated solution adhering to the inner walls for complete reaction
2. for titration result to be accurate

Question 5

suggest 1 change that could be made to reduce % error (context: low vol of titrant used)

Answer 5

1. use (titrant) of lower concentration
2. for volume of (titrant) used for each titration to be higher

Question 6

why do we add starch? and why isit added only near the end-point? (context: I2 and this late titration)

Answer 6

1. makes colour change at end-point more obvious from blue-black to colourless
2. VS pale yellow to colourless which is not as distinct
3. [I2] is relatively high at the start of titration
4. if indicator added at the start, many I2 molecules will be trapped in starch molecules
5. since liberation of I2 molecules from starch molecules take time
6. titration results will be inaccurate

Question 7

explain why the standard solution of NAOH(aq) cannot be prepared using NAOH(s) pellets

Answer 7

1. NAOH pellets are hygroscopic (absorb moisture from air)
2. pellets may be coated with a layer of Na2CO3 when reacted with atmospheric CO2 (*other hydroxides like Ba(OH)2 can also react with acidic atmosphere CO2 to give Ba2CO3: inaccurate results)
3. mass of NAOH cannot be determined accurately

Question 8

some sodium ethanedioate formed during titration appeared as white ppt in conical flask. explain whether this affected the accuracy of your results. (context: titration of (COOH)2 and NaOH)

Answer 8

1. cause solution to turn slightly cloudy
2. affect judgement of colour change at end point
3. affect accuracy of titration

Question 9

explain the need to carry out titration immediately once iodine is produced in conical flask (context: iodate to iodine, titration involved iodine + thiosulfate)

Answer 9

1. I2 is non-polar + poor solubility in water
2. some I2 will escape to surroundings if aq sol was left standing and not titrated immediately
3. titre value will be lower than actual value