integralcalcer1235
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I don't understand the answer, if the equivalence point is 6.5 shouldn't bromthymol blue be used?
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help for indicator question (1 Viewer)
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I don't understand the answer, if the equivalence point is 6.5 shouldn't bromthymol blue be used?
thush@decode
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You raise a very fair point. Based on the graph it would appear as though bromothymol blue would be a more appropriate indicator to use in comparison to methyl orange whose pKa is about 3.5 (the indicator colour would be orange at this point). I'd actually have to agree with you here - based on the graph that is provided to you.
In almost all cases, you'd expect methyl orange to be the go-to-indicator for strong acid-weak base titrations. In fact, I am not so sure whether this HCl/Na2CO3 titration would actually yield a titration curve like that; I'd expect the equivalence point pH to be more in the vicinity of 3.5-4 (perfect for methyl orange!) rather than the 6.5.
So I would contend that the pH curve does not really match the titration given in the question and, therefore, the sample answer (whilst matching the titration itself) does not really match the pH curve well at all.
integralcalcer1235
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hmmm would there be no such case that this occurs? like maybe it’s done at a specific temperature that affects Ka and Kb, if i were in an exam assuming they provided a graph such as that i should be reading equivalence point and thus determining the equivalence point from there and not just generalising a weak base and strong acid to be methyl orange even though it should be right? cause if they provide a graph i’m assuming i should use it
Where is this from?
This question is flawed, including for reasons noted above.
integralcalcer1235
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It's from Total Education Centre Trial 2020, could you explain what the correct answer is? Is it Bromthymol Blue?
Given any titration curve, a suitable indicator is one whose pH range is entirely within the "vertical" section of the curve, and ideally where the pH range includes the pH at the equivalence point.
For this curve, and going by eye, bromothymol blue (pH range 6.0 to 7.6) would be a suitable indicator.
If no table of indicators was given, I would accept methyl red (pH range 4.8 to 6.0) and phenol red (pH range 6.8 to 8.4) as these also fall into the vertical range of the pH curve.
I would not give an answer at all along the lines of the one provided (I presume) from the solutions, for several reasons:
Firstly, most titration curves are given for 1:1 reactions. For non-1:1 case, there are typically multiple end points. For example, if I titrated 15.00 mL of 0.1000 M phosphoric acid (conical flask) with 0.1000 M sodium hydroxide, I would get equivalence points at 15.00 mL, 30.00 mL, and 45.00 mL, corresponding to the reactions:
H3PO4 + NaOH ----> NaH2PO4 + H2O
NaH2PO4 + NaOH ----> Na2HPO4 + H2O
Na2HPO4 + NaOH ----> Na3PO4 + H2O
The pHs at the equivalence points would be at something like 4, 9.5, and 13, reflecting that the dihydrogenphosphate ion is a weak acid and that the hydrogenphosphate and phosphate ions are weak bases.
In the case of sodium carbonate, adding a strong acid can protonate to the hydrogencarbonate ion and again to carbonic acid, so there are two stages. But, carbonic acid exists in equilibrium with carbon dioxide and water, so the system in the question appears to me to be mixing the sought reaction
Na2CO3(aq) + HCl(aq) ----> NaHCO3(aq) + NaCl(aq)
with the alternative
Na2CO3(aq) + 2 HCl(aq) ----> 2 NaCl(aq) + CO2(g) + H2O(l)
It is my guess that the graph given is not based on empirical data but rather a simulation (you can see how the graph has joined plotted points with straight lines). That simulation would use the pKa values and ignore the likely production of CO2 gas to get a theoretical result that is flawed due to relevant factors being neglected. I doubt a teacher would ever get students to titrate an acid into a carbonate solution for exactly this reason. And this graph shows adding carbonate ions into a solution where hydronium ions are initially in massive excess, and asking that only the protonation occurs! Not likely.
For this curve, and going by eye, bromothymol blue (pH range 6.0 to 7.6) would be a suitable indicator.
If no table of indicators was given, I would accept methyl red (pH range 4.8 to 6.0) and phenol red (pH range 6.8 to 8.4) as these also fall into the vertical range of the pH curve.
I would not give an answer at all along the lines of the one provided (I presume) from the solutions, for several reasons:
- Methyl orange, with its pH range of 3.2 to 4.4, is obviously unsuitable for the given system based on the pH curve provided
- Discussing the pH of the equivalence point by reference to the nature of the acid and base used is simply poor technique. The equivalence point occurs when the acid and base are present in their stoichiometric ratio. Thus, it represents the point where the acid and base have (theoretically) completely reacted and so are no longer present in the system - so why would they determine the pH? The answer is, they wouldn't and don't.
- The substances that are present at the equivalence point are the salt formed and water, and so the pH should be rationalised in terms of those substances. So, for example, suppose we had a strong acid / weak base titration like nitric acid and ammonia. At the equivalence point, the salt present is ammonium nitrate and it is a weak acid as the hydrolysis (reaction with water) of the ammonium ion produces some hydronium ions which cause the pH to decrease, via: NH4+(aq) + H2O(l) <----> NH3(aq) + H3O+(aq) The nitrate ion, being the conjugate base of a strong acid, does not hydrolyse in water to any significant extent and so the pH at the equivalence point is in the acidic range solely because of the weak acid nature of the ammonium ion.
- You may notice that, in (3), I did not choose the system in this question. That is for a very good reason... it is not clear what the system is!
Firstly, most titration curves are given for 1:1 reactions. For non-1:1 case, there are typically multiple end points. For example, if I titrated 15.00 mL of 0.1000 M phosphoric acid (conical flask) with 0.1000 M sodium hydroxide, I would get equivalence points at 15.00 mL, 30.00 mL, and 45.00 mL, corresponding to the reactions:
H3PO4 + NaOH ----> NaH2PO4 + H2O
NaH2PO4 + NaOH ----> Na2HPO4 + H2O
Na2HPO4 + NaOH ----> Na3PO4 + H2O
The pHs at the equivalence points would be at something like 4, 9.5, and 13, reflecting that the dihydrogenphosphate ion is a weak acid and that the hydrogenphosphate and phosphate ions are weak bases.
In the case of sodium carbonate, adding a strong acid can protonate to the hydrogencarbonate ion and again to carbonic acid, so there are two stages. But, carbonic acid exists in equilibrium with carbon dioxide and water, so the system in the question appears to me to be mixing the sought reaction
Na2CO3(aq) + HCl(aq) ----> NaHCO3(aq) + NaCl(aq)
with the alternative
Na2CO3(aq) + 2 HCl(aq) ----> 2 NaCl(aq) + CO2(g) + H2O(l)
It is my guess that the graph given is not based on empirical data but rather a simulation (you can see how the graph has joined plotted points with straight lines). That simulation would use the pKa values and ignore the likely production of CO2 gas to get a theoretical result that is flawed due to relevant factors being neglected. I doubt a teacher would ever get students to titrate an acid into a carbonate solution for exactly this reason. And this graph shows adding carbonate ions into a solution where hydronium ions are initially in massive excess, and asking that only the protonation occurs! Not likely.
thush@decode
New Member
Agree with everything CM_Tutor has said above.
Rest assured that in the real HSC exam, such inaccuracies and inconsistencies would be extremely rare so you would not be expected to try and navigate questions like these. HSC exams (from what I can see on the NESA website) are vetted extremely stringently by teachers, subject experts in the field, literacy specialists and is even sat by a practising HSC teacher to pick up issues like this.
Rest assured that in the real HSC exam, such inaccuracies and inconsistencies would be extremely rare so you would not be expected to try and navigate questions like these. HSC exams (from what I can see on the NESA website) are vetted extremely stringently by teachers, subject experts in the field, literacy specialists and is even sat by a practising HSC teacher to pick up issues like this.
Unfortunately, the same cannot be said for practice questions found online or even in published books, of school assessments, and even of the trials.
Students need to be willing to consider that a question may be flawed and to consider how to respond appropriately in that event. In particular, if a student is sure that an answer that they get is right even though it does not match the question, do not attempt to fudge your way to the wrong answer!
thush@decode
New Member
Fair call. Even with my own published book (the Decode HSC Chemistry book) I am very much on the lookout for student and teacher feedback about any questions that may be flawed - ultimately you need many sets of eyes to pick out minor issues. So far, so good - but this particular thread underscores the importance of ensuring HSC topic test and trial exam questions are accurate.
There's nothing worse for an author/publisher than for a student to perform worse on their school assessments and HSC exams as a result of the trial exam/topic test.
There's nothing worse for an author/publisher than for a student to perform worse on their school assessments and HSC exams as a result of the trial exam/topic test.
Yes @thush@decode, feedback is vital both for finding actual mistakes and for finding those areas where the reader makes interpretations that are inconsistent with what the writer intended.
It takes a particular mindset to be good at spotting potential problems and actual mistakes. I recall when the uni I was working in was evaluating new textbooks for first year and the e-systems / questions available to students. With the online materials, most academics tried a few questions and put in the correct answers and were positive that these were accurately assessed as correct... I put in possible mistaken answers to see how the system responded. In some cases the systems were far from impressive.
There is an inherent problem with any published book that e-systems have the opportunity to handle better (though they rarely do) and that is that a single response is provided that needs to suit ever student / reader, which is impossible as learners are individuals. With face-to-face teaching, responses and advice can be tailored to the needs of the individual. For example, a textbook can be written to suit a broad range of students by including graded questions but the presentation of theory can't seek to address existing misconceptions or choose between multiple approaches to explaining a single concept. The decode books (from what I have seen from samples) seek to provide greater detail in explanations, which suits those who are struggling but risks being less useful for students of high ability. This is not a criticism - every author must make choices for which there can be no universal "correct" answer - but I think it is worth considering in the context of the potential for exam questions to be unclear or problematic.
It takes a particular mindset to be good at spotting potential problems and actual mistakes. I recall when the uni I was working in was evaluating new textbooks for first year and the e-systems / questions available to students. With the online materials, most academics tried a few questions and put in the correct answers and were positive that these were accurately assessed as correct... I put in possible mistaken answers to see how the system responded. In some cases the systems were far from impressive.
There is an inherent problem with any published book that e-systems have the opportunity to handle better (though they rarely do) and that is that a single response is provided that needs to suit ever student / reader, which is impossible as learners are individuals. With face-to-face teaching, responses and advice can be tailored to the needs of the individual. For example, a textbook can be written to suit a broad range of students by including graded questions but the presentation of theory can't seek to address existing misconceptions or choose between multiple approaches to explaining a single concept. The decode books (from what I have seen from samples) seek to provide greater detail in explanations, which suits those who are struggling but risks being less useful for students of high ability. This is not a criticism - every author must make choices for which there can be no universal "correct" answer - but I think it is worth considering in the context of the potential for exam questions to be unclear or problematic.
thush@decode
New Member
Mmm - I get your point. Our aim was to try cater for students of ranging abilities which I definitely found a challenge.
The way we approached it was to have a graded approach to questions - exam questions tend to be a little harder than topic test questions, and (at least for HSC Chemistry, Mathematics Advanced and Maths Extension 1) the exams are graded in order of difficulty - and a slight difference in flavour of the Detailed Solutions (the section where you find the detailed explanations you were referring to) as you move from easier to harder questions, and from topic tests to trial exams.
Ultimately, there is no better resource than face-to-face teaching - precisely for the reasons you have outlined above. That said, ideally our study guides make self-directed learning and exam/assessment revision much easier, and the acquired background knowledge would render face-to-face teaching episodes more efficient and enjoyable for both student and teacher.
@thush@decode, I have only seen the samples of the books so could well have a skewed impression. Catering for a range of abilities in a written format is indeed a challenge, and I wonder if I am alone in wondering how the more difficult material is presented. Have you considered including a sample of how the more difficult questions are handled?
Scaffolding that is beneficial for students first learning a topic or who are struggling can be less helpful for those whose understanding is better. It can even become an impediment. This occurs both in questions and in solutions. The example at the start of this thread provides one example - where the stimulus material is not necessarily helpful. Had the question not specified the acid or base, the ambiguity would have been avoided. Another example that a recently encountered was a question along the following lines:
A hydrate of ammonium iron(II) sulfate is dissolved in water. A solution of barium chloride is added until precipitation of barium sulfate is complete, and the product is collected and dried. The following data was provided:
Put another way, structure (in a question or in a set of solutions) that can be supportive for one student can interfere with the autonomy of another who can see alternative approaches. In writing a book, it is inevitable that choices are made and many of them are judgement calls where reasonable people can differ.
Scaffolding that is beneficial for students first learning a topic or who are struggling can be less helpful for those whose understanding is better. It can even become an impediment. This occurs both in questions and in solutions. The example at the start of this thread provides one example - where the stimulus material is not necessarily helpful. Had the question not specified the acid or base, the ambiguity would have been avoided. Another example that a recently encountered was a question along the following lines:
A hydrate of ammonium iron(II) sulfate is dissolved in water. A solution of barium chloride is added until precipitation of barium sulfate is complete, and the product is collected and dried. The following data was provided:
- the mass of ammonium iron(II) sulfate dissolved
- the formula of ammonium iron(II) sulfate was given as Fe(NH4)2(SO4)2.xH2O
- the mass of barium sulfate collected after drying
- Find the chemical amount of sulfate present in the precipitate
- Find the chemical amount of sulfate present in the original sample ammonium iron(II) sulfate
- Find the chemical amount of iron cations present in the original sample ammonium iron(II) sulfate
- Find the chemical amount of ammonium ions present in the original sample ammonium iron(II) sulfate
- Find the mass of (a) iron, (b) ammonium, and (c) sulfate present in the original sample ammonium iron(II) sulfate
- Find the value of x
Put another way, structure (in a question or in a set of solutions) that can be supportive for one student can interfere with the autonomy of another who can see alternative approaches. In writing a book, it is inevitable that choices are made and many of them are judgement calls where reasonable people can differ.