ATAR Notes: Forum
VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: ggxoxo on February 25, 2012, 11:38:52 am
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Hi guys; can youn please answer some of my questions
QUESTION 72
In a titration, an acid of known concentration is placed in a burette and reacted with a base
that has been pipetted into a conical flask. What should each piece of glassware be rinsed
with immediately before the titration?
Burette Pipette Conical Flask
A- Acid Base Water
B- Water Water Water
C- Acid Base Base
D- Water Water Base
(The answer is A)- what I don't get is wy the conical flask needs to be rinsed with water when the base is what is eventually placed in there
QUESTION 69
Sulfuric acid (H ,S0 4) and nitric acid (HNOJ) are both strong acids. Ethanoic acid
(CH ,COOH ) is a weak acid. 20.001111 solutions of 0.10 M concentration of each of these
three acids were separately titrated with a 0.10 M solution of sodium hydroxide (NoOH). In
order to react completely:
A All three acids would require the same amount of NoOH .
B HNO, would require more NoOH than CH JCOOH but less than H ,S04.
C H ,S04 and HNO, would require the same amount of NoOH but CH JCOOH would
require less.
D CH,COOH and HNO, would require the same amount of NaOH but H2S04 would
require more.
(The answer is D)- again what I don't get is HNO3 is stronger than CH3COOH so why do they need the same amount of NaOH when there will be more H+ ions in the HNO3 soln compared to the CH3COOH soln.
Thank you!!!
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Hi guys; can youn please answer some of my questions
QUESTION 72
In a titration, an acid of known concentration is placed in a burette and reacted with a base
that has been pipetted into a conical flask. What should each piece of glassware be rinsed
with immediately before the titration?
Burette Pipette Conical Flask
A- Acid Base Water
B- Water Water Water
C- Acid Base Base
D- Water Water Base
(The answer is A)- what I don't get is wy the conical flask needs to be rinsed with water when the base is what is eventually placed in there
QUESTION 69
Sulfuric acid (H ,S0 4) and nitric acid (HNOJ) are both strong acids. Ethanoic acid
(CH ,COOH ) is a weak acid. 20.001111 solutions of 0.10 M concentration of each of these
three acids were separately titrated with a 0.10 M solution of sodium hydroxide (NoOH). In
order to react completely:
A All three acids would require the same amount of NoOH .
B HNO, would require more NoOH than CH JCOOH but less than H ,S04.
C H ,S04 and HNO, would require the same amount of NoOH but CH JCOOH would
require less.
D CH,COOH and HNO, would require the same amount of NaOH but H2S04 would
require more.
(The answer is D)- again what I don't get is HNO3 is stronger than CH3COOH so why do they need the same amount of NaOH when there will be more H+ ions in the HNO3 soln compared to the CH3COOH soln.
Thank you!!!
1. The conical flask is rinsed with water because it is important that only the base from the pipette is added to the flask and not any residual drops still in the flask. Although residual drops of water may remain in the flask and will eventually dilute the base added from the pipette, this will not change results as the amount of base (in moles) will be the same, and (because it is delivered by the pipette) a known volume of the base is added.
2. Sulfuric acid, (H2SO4) is a diprotic acid. It can donate a total of 2 protons (it doesn't really, and the extent to which this happens can be calculated, but for now this should be sufficient) whereas both acetic acid (CH3COOH) and nitric acid (HNO3) can only donate a single proton. Therefore, more NaOH is required to neutralise the sulfuric acid.
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So for Qn 2- is it right to say that 'CH,COOH and HNO, would require the same amount of NaOH' or does HNO3 require a bit more NaOH than CH3COOH because there will be more H+ ions in HNO3 per volume compared to CH3COOH right?
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It's reacting with a strong base, the reaction goes to completion.
even though CH3COOH is a weak acid, it will donate all its protons in the presence of NaOH
likewise with strong acids, i.e. HNO3
so yes they both require the same amount
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Hi guys; I've got another question- how do you choose an indicator? Could you guys please give me like some sort of 3 golden rules or something of that sort? Also, do you tend to pick the end point after or before the equivalence point?
Thank you!!! :)
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Well generally, if it is a strong base and weak acid your ph will be higher than 7, so you want an indicator that changes colour in that approximate range
similarly weak base strong acid ph will be lower than 7
strong base strong acid will require an indicator which changes colour at around about 7
Generally though, I'm pretty sure, you'll be told, or given an indication of around what ph the equivalence point is. Based on that data, select an indicator that changes colour at that value; that is, an indicator which fits in the range of the equivalence point
Also, your endpoint is always after your equivalence point as that it the point after it is in stoichiometric ratios. If your end point was before your equivalence point, you would never know how much more you would need to find the stoichiometric ratios, and hence calculations would be inaccurate.
Sorry if any of this was too general :S
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Well generally, if it is a strong base and weak acid your ph will be higher than 7, so you want an indicator that changes colour in that approximate range
similarly weak base strong acid ph will be lower than 7
strong base strong acid will require an indicator which changes colour at around about 7
Generally though, I'm pretty sure, you'll be told, or given an indication of around what ph the equivalence point is. Based on that data, select an indicator that changes colour at that value; that is, an indicator which fits in the range of the equivalence point
Also, your endpoint is always after your equivalence point as that it the point after it is in stoichiometric ratios. If your end point was before your equivalence point, you would never know how much more you would need to find the stoichiometric ratios, and hence calculations would be inaccurate.
Sorry if any of this was too general :S
and i thiink the data booklet has the information you need for indicators too, so if you know the range of equivalence point,using data booklet you could figure it out ,
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Well generally, if it is a strong base and weak acid your ph will be higher than 7, so you want an indicator that changes colour in that approximate range
similarly weak base strong acid ph will be lower than 7
strong base strong acid will require an indicator which changes colour at around about 7
Generally though, I'm pretty sure, you'll be told, or given an indication of around what ph the equivalence point is. Based on that data, select an indicator that changes colour at that value; that is, an indicator which fits in the range of the equivalence point
Also, your endpoint is always after your equivalence point as that it the point after it is in stoichiometric ratios. If your end point was before your equivalence point, you would never know how much more you would need to find the stoichiometric ratios, and hence calculations would be inaccurate.
Sorry if any of this was too general :S
No it was good- exactly what I was looking for; thank you- but I still need some clarifications- if the end point was after the equivalence point, wouldn't you still get an incorrect answer, because then you would get an overestimate?
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No it was good- exactly what I was looking for; thank you- but I still need some clarifications- if the end point was after the equivalence point, wouldn't you still get an incorrect answer, because then you would get an overestimate?
Yeah, you're right, but the important thing is that if you've picked your indicator correctly, the endpoint would be at most one or two drops past the equivalence point, which fits in the error of (+-) 0.05 ml for the burette, and would not play a significant impact on results. Whereas if it was before, you would not know how far before equivalence you've stopped it..
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Thank you!!!
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Hi guys new question; my SAC is tomorrow and it's a titration. I was just wondering what would be good things to put down to improve the design of the experiment?
Any suggestions is fine- let's brainstorm!!!
Thank you!!!
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acid/base or redox?
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just finished it nvr mind!!! it was acid/base; redox is in 2 weeks!
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Hey guys; why do we standardise compounds? I still don't get why we need secondary standards. For example, why do we standardise NaOH (aq)?
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Sometimes the initial (or primary) standard might change from the time it is prepared, through decomposition, reaction to sunlight, etc. Therefore by testing them with a second standard (which you know the concentration of) you can confirm the characteristics of the primary standard and then use that to solve the unknown.
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So just to get this straight- so we don't use it to lower the concentration of the primary standard because it is so strong that we have to dilute to a weaker form that is suitable in a classroom setting???
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I don't know if I'm missing something here but if NaOH is not a good primary standard, instead of standardising it, why can't we just use a different base that is actually a good primary standard?
Also, what would happen to measurement if the tip of the burette was not filled with liquid before doing the titration?
Thank you!!!
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Also, what would happen to measurement if the tip of the burette was not filled with liquid before doing the titration?
You would be titrating less volume than the readings are telling you (some of the solution will fill the tip, and not mix with the other solution). This would result in you calculating a higher concentration for the unknown.