Stuck on some practical questions on titration HELP! Watch

Yatayyat
Badges: 14
Rep:
?
#1
Report Thread starter 1 month ago
#1
There are 3 MCQ that I am stuck on and struggling to answer.

https://imgur.com/SopaflL
https://imgur.com/gF73C7W

Q 20)

I'm thinking it might be option A.The judgements that I have tried to draw out here is that when inverting a volumetric flask multiple times, it ensures that the solute and solvent mixes thoroughly and evenly in the solution. I know that this is in the earlier parts of the titration process when you need to prepare a standard solution. So I'm thinking that inverting the flask several times is crucial because it ensures that the concentration is evenly distributed, and this is important because for standard solutions the concentration in it must be accurately known, since we usually pipette out a portion of the standard solution yet the concentration still must be kept constant.

It can't be option B because washing the pipette with distilled water will in fact dilute the acid even more; since this pipette is used to transfer the acid into the conical flask. If that is the case then I think it will result in a smaller titre value from the burette. This is due to it being that less volume of the sodium hydroxide will be needed to fully neutralised the acid. Though I can't see what change it will have to achieve concordant titres?

It can't be option C because adding more indicator drops doesn't really change the number moles of acid or alkali that react together, so won't affect the titre values.

For option D, if a more concentrated value of sodium hydroxide is used then this will also lead to a smaller titre value because we have more moles per unit volume of sodium hydroxide so less of it will be needed to fully neutralise the acid in the conical flask. Again I can't see how it has an effect on concordant results?

Q 21)

I don't even know what the answer could be? Though this is what I have thought.

With option A, if I have rinsed the flask with acid then I am ultimately increasing the number of moles of acid in the conical flask. Surely this will mean more NaOH has to be used to neutralise the acid, but wouldn't this totally ruin the experiment because you are inputting more acid from nowhere.

Similarly if it is rinsed with alkali then the acid will become more dilute and less of NaOH is used.

Now rinsing with water, I think has no effect in improving accuracy of titration. That is since adding water will dilute the concentration more but doesn't really change the moles of acid in the conical flask. Because we know how much moles of acid there was before from taking a fixed volume of our standard solution where the molarity of it was also known too. Or I'm I mistaking myself into thinking that adding water has actually a different purpose in the titration. If yes then I'm assuming distilled water is used to wash off the inside walls of the conical flask. I think this is done to ensure all the moles of alkali that were given off from the burette react with the acid.

Q 22)

In option A, a less concentrated alkali in the burette will give a larger titre value. I think that will give a smaller percentage uncertainty, since to calculate uncertainty for the burette measurement it is: 2 *instrument uncertainty/measured titre value

If the measured titre value is bigger then the percentage uncertainty will get smaller indeed.

For option B, I don't see how using a different colour indicator will effect uncertainty

For option C, I don't know what effect it will have, so could someone please tell me what would happen here.

And for option D, starting at the zero mark on the burette will still have the same percentage uncertainty as opposed to starting at a mark that isn't zero.

These are what I have been thinking so far, but could anyone help in telling me what the answer could be and why the other options would be wrong.

Any help would be really appreciated! Thanks!
Last edited by Yatayyat; 1 month ago
0
reply
BobbJo
Badges: 12
Rep:
?
#2
Report 1 month ago
#2
(Original post by Yatayyat)
There are 3 MCQ that I am stuck on and struggling to answer.

https://imgur.com/SopaflL
https://imgur.com/gF73C7W

Q 20)

I'm thinking it might be option A.The judgements that I have tried to draw out here is that when inverting a volumetric flask multiple times, it ensures that the solute and solvent mixes thoroughly and evenly in the solution. I know that this is in the earlier parts of the titration process when you need to prepare a standard solution. So I'm thinking that inverting the flask several times is crucial because it ensures that the concentration is evenly distributed, and this is important because for standard solutions the concentration in it must be accurately known, since we usually pipette out a portion of the standard solution yet the concentration still must be kept constant.

It can't be option B because washing the pipette with distilled water will in fact dilute the acid even more; since this pipette is used to transfer the acid into the conical flask. If that is the case then I think it will result in a smaller titre value from the burette. This is due to it being that less volume of the sodium hydroxide will be needed to fully neutralised the acid. Though I can't see what change it will have to achieve concordant titres?

It can't be option C because adding more indicator drops doesn't really change the number moles of acid or alkali that react together, so won't affect the titre values.

For option D, if a more concentrated value of sodium hydroxide is used then this will also lead to a smaller titre value because we have more moles per unit volume of sodium hydroxide so less of it will be needed to fully neutralise the acid in the conical flask. Again I can't see how it has an effect on concordant results?

Q 21)

I don't even know what the answer could be? Though this is what I have thought.

With option A, if I have rinsed the flask with acid then I am ultimately increasing the number of moles of acid in the conical flask. Surely this will mean more NaOH has to be used to neutralise the acid, but wouldn't this totally ruin the experiment because you are inputting more acid from nowhere.

Similarly if it is rinsed with alkali then the acid will become more dilute and less of NaOH is used.

Now rinsing with water, I think has no effect in improving accuracy of titration. That is since adding water will dilute the concentration more but doesn't really change the moles of acid in the conical flask. Because we know how much moles of acid there was before from taking a fixed volume of our standard solution where the molarity of it was also known too. Or I'm I mistaking myself into thinking that adding water has actually a different purpose in the titration. If yes then I'm assuming distilled water is used to wash off the inside walls of the conical flask. I think this is done to ensure all the moles of alkali that were given off from the burette react with the acid.

Q 22)

In option A, a less concentrated alkali in the burette will give a larger titre value. I think that will give a smaller percentage uncertainty, since to calculate uncertainty for the burette measurement it is: 2 *instrument uncertainty/measured titre value

If the measured titre value is bigger then the percentage uncertainty will get smaller indeed.

For option B, I don't see how using a different colour indicator will effect uncertainty

For option C, I don't know what effect it will have, so could someone please tell me what would happen here.

And for option D, starting at the zero mark on the burette will still have the same percentage uncertainty as opposed to starting at a mark that isn't zero.

These are what I have been thinking so far, but could anyone help in telling me what the answer could be and why the other options would be wrong.

Any help would be really appreciated! Thanks!
20. I would agree with A.

21. You only rinse a conical flask with water, not acid or alkali because residual acid and alkali will affect your titre. You wash with water so that it removes the residual mixture remaining after titration so C

22. I would agree with A.
C would give you a smaller titre with greater percentage uncertainty
0
reply
Yatayyat
Badges: 14
Rep:
?
#3
Report Thread starter 1 month ago
#3
(Original post by BobbJo)
20. I would agree with A.

21. You only rinse a conical flask with water, not acid or alkali because residual acid and alkali will affect your titre. You wash with water so that it removes the residual mixture remaining after titration so C

22. I would agree with A.
C would give you a smaller titre with greater percentage uncertainty
Thanks for replying! So I seem to be fine with q 20 and 22, but with question 21 how does removing the residual mixture into the solution itself actually improve the titration accurcay when we rinse the residual mixture with distilled water?
Last edited by Yatayyat; 1 month ago
0
reply
BobbJo
Badges: 12
Rep:
?
#4
Report 1 month ago
#4
(Original post by Yatayyat)
Thanks for replying! So I seem to be fine with q 20 and 22, but with question 21 how does removing the residual mixture into the solution itself actually improve the titration accurcay when we rinse the residual mixture with distilled water?
In this case, we are titrating the acid with NaOH. At the end, there may be residual NaOH. We have to wash the flask with water to remove it or it will affect the next titre value.

you should be washing the conical flask with water in between titrations.
0
reply
Yatayyat
Badges: 14
Rep:
?
#5
Report Thread starter 1 month ago
#5
(Original post by BobbJo)
In this case, we are titrating the acid with NaOH. At the end, there may be residual NaOH. We have to wash the flask with water to remove it or it will affect the next titre value.

you should be washing the conical flask with water in between titrations.
Thanks this makes sense to me now!
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Cardiff University
    Undergraduate Open Day Undergraduate
    Wed, 27 Mar '19
  • University of Portsmouth
    Postgraduate and Part-Time Open Evenings Postgraduate
    Wed, 27 Mar '19
  • Middlesex University London
    Postgraduate Open Evening Postgraduate
    Wed, 27 Mar '19

How old were you when you first saw porn?

I've never seen it (144)
20.54%
Before I was 12 (239)
34.09%
13 (110)
15.69%
14 (84)
11.98%
15 (44)
6.28%
16 (22)
3.14%
17 (9)
1.28%
18 (6)
0.86%
Between the ages of 19 - 24 (11)
1.57%
Over 25 (2)
0.29%
12 (30)
4.28%

Watched Threads

View All