Practical 4

Practical 4

Experimental Procedurerecall

Before experiment starts

• Rinse all apparatus with water and distilled water

• Burette, pipette, conical flasks, graduated flask, beaker, dropper, glass rod, filter funnel

• Wipe beaker (for weighing) dry• Wipe exterior of pipette dry

Weighing

• TARE function may be used• Place beaker in balance, close the doors, press

TARE, wait for 0.000 to appear• TAKE OUT beaker to add solid• Place beaker back in balance, close the doors,

wait for the mass to stabilise

Repeat till mass obtained is what you need

Making standard solution

• Add abt 20 cm3 of distilled water to dissolve the crystals (a little more may be added if solid cannot dissolve)

• Place filter funnel on top of graduated flask• Use glass rod to guide solution from beaker

into graduated flask• Glass rod should be placed back into the

beaker

Making standard solution

• Rinse the walls of the beaker and the glass rod with distilled water

• Transfer these washings into the graduated flask

• Repeat above 2-3 times• Rinse the sides of the filter funnel with distilled

water• Add distilled water until a finger below the

mark

Making standard solution

• Remove filter funnel• Use dropper to add distilled water drop by

drop into the graduated flask till bottom of meniscus meets the line/mark

• Close the cap, shake the flask to ensure homogeneous solution obtained

Adding solution from graduated flask into conical flask

• Rinse the pipette with solution (Suck solution till about 2 fingers above the mark)

• Discard the solution

• Suck solution till slightly above the mark• Hold pipette tip above solution• Release solution using E button till the bottom

of meniscus touch the mark

• Wipe the exterior of pipette dry, avoid touching the tip of the pipette

• Release the solution into the conical flask• When end, twist the pipette against the

bottom of the conical flask (Do not try to get all the solution into the flask, as this vol of solution has been accounted for)

Adding NaOH into burette

• Place filter funnel on top of burette• Add a small amount of NaOH• Remove filter funnel• Rinse the wall of the burette• Rinse the burette tip by releasing the burette

clip

• Place filter funnel on top the burette• Add NaOH into the burette• Remove filter funnel• Release burette clip and ensure burette tip is

filled with solution• No bubbles found in burette• Read the initial volume

Practical 4

Precision of apparatus

Apparatus Precision RemarkPipette 1 dp

10.0 or 25.0Burette 2 dp

xx.x0 or xx.x5Last digit is 0 or 5

Volumetric/graduated flask

To 3 sf100 or 250

Electronic balance 0.xxx Conical flask Not for measurementMeasuring cylinder Low precision Usually use for reagents

that are added in excess

Practical 4

Evaluation

1

• Comment on the consistency of your titre values.

• Identify and explain the presence or absence of anomalous results in these titre values.

Situation 1

• There is no anomalous result as the results obtained are consistent to ±0.10 cm3

• Two reasons why (Pg 40)

• Note : the results includes the rough titration, and the rough titration is cancelled

Situation 2

• The 2 results chosen are consistent to ±0.10 cm3.

• Two reasons why

• Note : the results does not include the rough titration, and the rough titration is cancelled

2

• NaOH solid is not suitable for preparation of standard solution as it is deliquescent, explain how this will affect the concentration of the solution prepared and suggest ways to overcome it.

How it affect

• NaOH absorbs moisture from the air which will cause the mass measured to include the mass of water absorbed by NaOH solid. For the same mass weighed, a smaller amount of NaOH is obtained. The concentration of NaOH solution will be lower than expected.

Ways to overcome

• Weighing of the NaOH solid has to be carried out fast.

• Container used to store NaOH solid must be properly sealed to minimise contact with air.

3

• State one precaution that should be taken to ‘maintain’ the quality of the sodium hydroxide prepared.

Facts

• NaOH solution being basic can absorb CO2 from the air.

• Amount of NaOH present in the solution is lesser than expected.

• [NaOH] < expected

Precaution

• The NaOH solid weighed must be used immediately to prepare the solution.

• After the NaOH solution is prepared, ensure that it is always covered to minimise contact with air.

4

• Is there a more accurate way of determining the endpoint instead of the use of indicator?

Facts

• When colour changes from colourless (acid) to permanent pale pink (alkali), the equivalence point has been exceeded.Vol. of end point > Vol. of equivalence point

• The observation of colour change to detect endpoint of the titration can be subjective, and sometimes ambiguous.

More accurate way

• Use instrumental method– Use pH meter to monitor the pH value of the

solution, and plot a graph of pH against volume of titre

– Use data logger to plot the change in pH with the addition of solution form the burette. The endpoint correspond to the drastic change in pH during titration.

Practical 4

Explain the importance of the following steps:

i

• Ethanedioic acid crystals must be weighed accurately and the ethanedioic solution must be prepared in graduated flask.

i

• A more precise weighing balance will give a smaller percentage error in the mass of ethanedioic acid crystals weighed.

• A graduated flask measure volume with high accuracy, lower percentage error in the volume of the ethanedioic solution.

ii

• Use of pipette and burette to measure the volume of acid and NaOH respectively.

ii

• More precise apparatus is used to measure the volume of the solution decreases the percentage error of volume measured.

• Using a pipette to 1 dp, ensures that an accurate volume of 10 cm3 acid is obtained. An accurate amount of acid is present in the conical flask for titration.

• Using a burette to 2dp ensures that the volume of NaOH used is more accurately determined, and therefore a more accurate [NaoH]

iii

• Washings must be transferred into the graduated flask when the standard ethanedioic acid solution was being prepared.

iii

• Ensure all the ethanedioic acid that was not transferred into the graduated flask and present in the washings was transferred.

• This ensures that the amount of ethanedioic acid present in graduated flask = amount of ethanedioic acid weighed.

• It will give rise to an accurate volume of NaOH used and therefore, accurate [NaOH]

iv

• Use of indicator is necessary.

iv

• It is to provide a distinct colour change to determine the end point.

v

• Only 1-2 drops of indicator is required.

v

• Indicators are also weak acids/bases that can also take part in the titration

• Excessive use will cause significant reactions to occur with the indicators

• Titration values will be higher/lower than expected

• [NaOH] determined will be lower/higher than actual