1999 HSC Question 28 - Indicator Choice - Help (1 Viewer)

[salih08]

Use the table to answer parts (a), (b).


(a) The equivalence point in a titration between 0·10 mol L–1 HCl(aq) and
0·10 mol L–1 NH3(aq) occurs at about pH 5·5.
(i) Give ONE reason why phenolphthalein is not a suitable indicator.
(ii) Select an indicator
(b) From the data, suggest a reason why litmus is not widely used in volumetric
analyses.

How would you answer this question for a band 6?


Comments from marking centre:
This was the most poorly done question in the whole examination paper. The concepts tested,
explaining indicator choice and titration curve sketching, were not well understood by candidates.
This could possibly be attributed to the fact that this section of the syllabus has had little
examination coverage in the past.
(a) and (b) tested the concept of indicator choice. Many candidates failed to relate their responses
to the data, relying rather on general answers. The great majority of candidates were able to
select the correct indicator in part (ii) but many failed to explain the reason why it was the
most appropriate. A significant number of candidates confused equivalence point with endpoint.
Approximately a third of candidates indicated that litmus was unsuitable as an indicator
because it was ÔpaperÕ and seemed unaware that it could be obtained in liquid form.
Expression and incorrect scientific usage were causes for concern.

 

[salih08]

please say what u think about this answer and how i can improve it. (I am not gonna be lazy and ask others to do it for me.) thanks.

a)
i)The PH range of phenolphthaelein is 8.3-10 ,and does not lie within the steep section of the titration curve as the equivalence point is at a pH of 5.5 . Therefor a false endpoint will be concluded from the colour change, making the indicator unsuitable.

ii)Methyl Red because it has a pH range of 4.4-6.2, which lies on the steep section of the titration curve and the end point will be signaled by a distinct change of colour in the solution.

b) litmus changes colour from blue to green which are both dark colours and the end point can not be distinguished as the colours are similar ???(biggest guess)

 

[ibbi00]

Something along those lines..
B) Litmus is not widely used as an indicator for volumetric analysis as the pH range in which it changes colour according to the equivalence point is far too great to obtain a reasonable and accurate endpoint.

 

[salih08]

Something along those lines..
B) Litmus is not widely used as an indicator for volumetric analysis as the pH range in which it changes colour is far too great to obtain a reasonable and accurate endpoint.

thanks. have i answered a) well?

 

[ibbi00]

Yes. Although I'm not sure of the need to refer to the cruve for titration.

 

[MetroMattums]

Just make sure you have the words endpoint and equivalence point in both those answers and you'll be fine.

 

[iMAN2]

Use the table to answer parts (a), (b).


(a) The equivalence point in a titration between 0·10 mol L–1 HCl(aq) and
0·10 mol L–1 NH3(aq) occurs at about pH 5·5.
(i) Give ONE reason why phenolphthalein is not a suitable indicator.
(ii) Select an indicator
(b) From the data, suggest a reason why litmus is not widely used in volumetric
analyses.

a)
i)The PH range of phenolphthaelein is 8.3-10 ,and does not lie within the steep section of the titration curve as the equivalence point is at a pH of 5.5 . Therefor a false endpoint will be concluded from the colour change, making the indicator unsuitable.

The endpoint (the point when an indicator changes colour) does not lie near the equivalence point (when equal moles of acid and base have reacted). This is because the pH of the equivalence point does not fall in a narrow range of the indicator's endpoints. Thus a false point of a neutralisation reaction will be shown and thus the experiment will be invalid.
This can be slightly problematic if you have defined endpoint as 'the point just after the equivalence point' instead of as 'the point when an indicator changes colour'.

ii)Methyl Red because it has a pH range of 4.4-6.2, which lies on the steep section of the titration curve and the end point will be signaled by a distinct change of colour in the solution.

Methyl Red, as it's pH is suitable so that it's endpoints will lie close to the equivalence point, thus validly showing the correct point of an occurrence of a neutralisation reaction.

Something along those lines..
B) Litmus is not widely used as an indicator for volumetric analysis as the pH range in which it changes colour according to the equivalence point is far too great to obtain a reasonable and accurate endpoint.

The pH range over which the endpoint (point at which colour change) occurs is larger than other indicators and so the endpoint will lie further from the equivalence point, thus questioning the experiment's validity.