Advanced 'O' Level Stoichiometry Qns (for Garrick_3658)
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Garrick_3658 had sent me a msg. In a nutshell, he said :
[Quote] Is it possible for you to provide me with a simplified step-by-step answers for these questions that you posted? Well, even my school teachers are dumbfounded by these things. They even questioned its standard, whether is it of O Level calibre. [/Quote]
In a nutshell, I replied :
[Quote] These questions are do-able at both 'O' levels and 'A' levels, meaning while the difficulty or complexity standard is fairly high, there is no new concept or new mathematical formula that you need to learn beyond the 'O' level syllabus, in order to complete these questions.
And I give these questions to (and coach) my tuition students, both 'O' levels and 'A' levels. Indeed, some of my ('O' level) students remarked that some of their own teachers in school may have difficulty solving these questions. But they (my 'O' level students) could appreciate how attempting and eventually solving (albeit more often than not with my guidance and teaching) these questions, benefited them in terms of depth and scope of thought.
So to answer you, are these questions 'O' level standard? Yes and no. No, in that these are probably somewhat more challenging than most 'O' level questions you would come across in the exam. Yes, in that there is nothing new you need to learn (eg. 'A' level stuff), in order to solve these questions... other than challenging yourself to think a little deeper and see a little further, to see the forest for the trees, so to speak.
So, now that this is out of the way, it's up to you whether you think it worth your time to solve these questions, or perhaps (depending on the individual student) it might be wiser to spend your time on 'O' level TYS questions instead.
Perhaps I'd leave you with this recommendation : if your mastery of the basic foundation, ie. stuff the syllabus indicates you need to memorize, definitions, drawings, methods, etc; is not there yet, then you'd be better off cheonging the 'O' level TYS.
But if you find the TYS questions too easy for you and fancy a challenge (that is beyond the typical 'O' level standard, but still do-able at 'O' levels), then attempt my questions.
Finally, if you wish to attempt these questions, eg. the 2 questions you mentioned, and you would like my help, then make a new post on the forum, and we can discuss these questions. I won't type or post or send you the complete worked solution though, instead what I'd do, is comment, guide and hint, as you attempt these questions. So in the new posts that you'll be making shortly (assuming you're still interested in attempting these questions), share your thought process and attempted working along the lines of, "I've done these calculations so far, and have made these assumptions, and plan to solve it this way... are there any conceptual errors I made? Would it be correct for me to say that..." and so on.
1 Week to 'O' Levels. Make good.
[/Quote]So here are the 2 questions of interest to Garrick_3658.
3) A mixture of MgSO4.7H2O and CuSO4.5H2O is heated until a mixture of the anhydrous salts, is obtained. If 5.0g of the hydrated mixture when heated gives 3.0g of the anhydrous salts, calculate the % by mass of CuSO4.5H2O in the initial hydrated mixture.
4) When Fe and Fe3+ are mixed together, a reaction occurs in which Fe2+ is produced. What is the ratio of Fe to Fe3+ required to produce equal moles of Fe2+ and Fe3+ when the reaction is complete?
I'd now invite Garrick_3658 and any other interested* 'O' level students ('A' level students are also welcome) to attempt these questions here in this thread and to share your working here, or feel free to ask questions to confirm, verify, or clarify your concepts relevant to these questions.
*Note : As I msg'd Garrick_3658, if you're an 'O' level student who hasn't already memorized and mastered your basic foundations, then you're better off cheonging the 'O' level TYS instead of attempting these questions. Don't waste your time if you know you're not yet ready, you have 1 week left to your exams, make good!!!
Now, to kickstart this thread, here are a couple of hints for the 2 qns.
Q3 Hint : Let x and y be the no. of moles of the hydrated (or anyhydrous, up to you, no difference to no. of moles) copper(II) sulfate and magnesium sulfate, respectively. Next, based on the information provided by the question, write two equations in x and y, and solve simultaneously.
Q4 Hint : First, write the relevant half equations to obtain the balanced RedOx equation. Next, although not strictly required (ie. it's optional), it may help you to get a mental handle on the situation if you write out an ICF table - Initial (no. of moles), Change (in no. of moles), Final (no. of moles).
The Game's Afoot!
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Since I called this thread "Advanced 'O' Level Stoichiometry Qns", it wouldn't do justice unless I include what is known as "The Ultimate Stoichiometry Question" that is do-able at 'O' Levels. But be warned, although this question will serve excellently as an overall Stoichiometry revision question to help prepare you for your exams, but attempt it only if you're confident you've already mastered and successfully memorized everything else your 'O' level (Pure or Combined Science) syllabus requires you to (otherwise you're better off cheonging your TYS than attempting advanced questions like these).
Ready? Here...... we...... GO!!!
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Haha, alright. I haven't did my TYS, but generally my main problem would be on organic chem and mole concept. These aside...
xMgSO4.7H2O + yCuSO4.5H2O --> xMgSO4 + yCuSO4 + 12H2O (removed)
I'm assuming no tricks (such as decomposition of sulphates, I didn't learn)
120x + 126 + 160y + 90 = 5
120x + 160y = 3
Something's wrong, but it's plain weird here. Perhaps I haven't really grasped my basic mole concepts well...
I don't know anything about Initial Change Final, I wasn't taught that way in school.
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Originally posted by Garrick_3658:
Haha, alright. I haven't did my TYS, but generally my main problem would be on organic chem and mole concept. These aside...
xMgSO4.7H2O + yCuSO4.5H2O --> xMgSO4 + yCuSO4 + 12H2O (removed)
I'm assuming no tricks (such as decomposition of sulphates, I didn't learn)
120x + 126 + 160y + 90 = 5
120x + 160y = 3
Something's wrong, but it's plain weird here. Perhaps I haven't really grasped my basic mole concepts well...
I don't know anything about Initial Change Final, I wasn't taught that way in school.
(I'm going out now, and will only be back after midnight. I'll reply then or tomorrow morning. In the meantime, we encourage anyone else interested in attempting these questions, to share your working, ideas or attempts here. Feel free to discuss, and have fun!)Garrick, a couple of quick comments for now :
Your equation 120x + 160y = 3 is correct. You need one more correct equation in x and y, to solve simultaneous equations.
Your other equation : xMgSO4.7H2O + yCuSO4.5H2O --> xMgSO4 + yCuSO4 + 12H2O (removed) is wrong (the 12 H2O part), because such an equation did not take into account different number of moles of the two salts - if it did, you would get (something)x + (something)y = (no. of moles of water) which will not be 12.
>>> I don't know anything about Initial Change Final, I wasn't taught that way in school. <<<
Don't worry about what you were taught in school (different schools teach slightly different things anyway). Use this chance to learn something new (it's not actually anything new, it's just a new way of looking at things). Again, you don't actually have to use the ICF table to solve this problem if you don't want to, it just HELPS you to handle the problem a little more conveniently, if you think in terms of ICF (it's a useful tool or way of looking at stoichiometric processes, not a new concept or formula!).
About the "Initial Change Final (ICF)" table:
http://www.chm.davidson.edu/ronutt/che115/Icf/Icf_Solutions2.htm
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Originally posted by UltimaOnline:
Your other equation : xMgSO4.7H2O + yCuSO4.5H2O --> xMgSO4 + yCuSO4 + 12H2O (removed) is wrong (the 12 H2O part), because such an equation did not take into account different number of moles of the two salts - if it did, you would get (something)x + (something)y = (no. of moles of water) which will not be 12.
Shld be x(120+7(18)) + y(160+5(18)) = 5 ?
Theroy is mass = mole x Mr
This is really cool.
Regarding this simultaneous way of solving, can be used on any mole question? Like the principle of moments (anticlockwise= clockwise moments)
Edit: amended my careless mistake
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Originally posted by davidche:
Shld be x(126+7(18)) + y(160+5(18)) = 5 ?
>>> x(126+7(18)) + y(160+5(18)) = 5 <<<
Almost correct, except you seem to have made a silly careless calculation error.
>>> Like the principle of moments (anticlockwise= clockwise moments) <<<
That sounds like Physics. You could ask Mod Eagle about this in a Physics thread.
>>> Regarding this simultaneous way of solving, can be used on any mole question? <<<
Depends on the question. Case by case basis. But I always advise my students : Algebra is your friend; always feel free to use it whenever you recognize it's potential in a calculation question (regardless of your school teacher's method taught in class; never be limited by dogma.).
>>> This is really cool. <<<
Glad you're enjoying it. This is how academia should be - FUN!
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In regards to the anyhydrous vs hydrated salts qn, I think it's almost cracked by Garrick and David; only the maths part is left, which can be easily solved by any 'O' level math student. So to every 'O' level student reading this, go ahead and solve this question.
In regards to the Fe, Fe2+, Fe3+ question, I'll wait for a student (eg. Garrick) to correctly write out the half equations and/or the balanced RedOx equation, before I continue advising on applying the ICF table.
In regards to the Ultimate Stoichiometry Qn, I'll wait for students to attempt this question, share their working here, and ask questions on it (eg. "I've solved it! is the final answer ___?" or "What the @#$% is an aliquot?!?"), before I make further comments on it.
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Roger. I managed to get somewhere until I got stucked, anyway.
Fe + 2Fe3+ --> 3Fe2+
Looks weird, is it correct?
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Originally posted by Garrick_3658:
Roger. I managed to get somewhere until I got stucked, anyway.
Fe + 2Fe3+ --> 3Fe2+
Looks weird, is it correct?
>>> Fe + 2Fe3+ --> 3Fe2+ <<<Yes, that is correct. Once you write the half-equations correctly, it is easy to write the overall balanced RedOx equation (students who try to short-cut to write the overall balanced redox without first writing the half-equations often end up with a wrongly balanced redox equation).
Let the required ratio of Fe to Fe3+ be 1 : x.
So you have Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change ??? ??? ???
Final ??? ??? ???
Complete the ICF table. Remember that for "Change", you need so apply STOICHIOMETRY. So start by using up your Fe.
So you have Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change -1 -(???) +(???)
Final ??? ??? ???
For Final, simply add up the Initial + Change, makes sense to you, right? I think you get the idea. Have fun!
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Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change -1 -y 3
Final 0 3 3 (from equation)
so x - y = 3
Ok stuck. But I know I have to use something from the equation (mole ratio of Fe3+?) but I don't seem to be able to GRASP the concept as to WHY to use the mole ratio...
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Originally posted by Garrick_3658:
Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change -1 -y 3
Final 0 3 3 (from equation)
so x - y = 3
Ok stuck. But I know I have to use something from the equation (mole ratio of Fe3+?) but I don't seem to be able to GRASP the concept as to WHY to use the mole ratio...
Your filled in values are incorrect. First, where did the 'y' variable come from? There should only be 'x', used in the required ratio of 1 : x (for Fe : Fe3+)By stoichiometry, if you use up 1 mole of Fe, how many moles must you use for Fe3+? Look at the stoichiometry. It's certainly not 'y' (what's 'y' anyway?).
Also note that for change, you need to include the positive or negative sign. You didn't include sign for the "change" in "product".
Your next mistake is, if for Fe3+, you initially had x, then you took away y (ie. -y), your final should be (x - y), but you somehow wrote "3" instead, I know it's because you tried to fit in "same no. of moles of Fe3+ and Fe2+"; don't do that. Write in the table values for Final based ONLY on Initial and Change; AFTER the table is complete, THEN apply the question info of "same no. of moles of Fe3+ and Fe2+" to come up with an equation in x, hence to solve for x, and therefore to answer the question.
You're very close to the answer, try again?
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Oh ok I get it now.
Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change -1 -2 +3
Final 0 x-2 3 (from equation)
x-2 = 3
x=5
The answer is 1:5, isn't it?
Thanks for all the help here, UltimaOnline. These knowledge shall serve me well for the upcoming O level Chemistry Exam. I envy your students. They are sure lucky to have a patient and knowledgeable teacher like you.
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Originally posted by Garrick_3658:
Oh ok I get it now.
Fe + 2Fe3+ ---> 3Fe2+
Initial 1 x 0
Change -1 -2 +3
Final 0 x-2 3 (from equation)
x-2 = 3
x=5
The answer is 1:5, isn't it?
Thanks for all the help here, UltimaOnline. These knowledge shall serve me well for the upcoming O level Chemistry Exam. I envy your students. They are sure lucky to have a patient and knowledgeable teacher like you.
Good job Garrick, your answer of 1:5 is correct.Would you like to attempt the "Ultimate Stoichiometry Challenge" Qn? And you mentioned Organic Chem as your other topic you have questions on, so feel free to ask away.
And Thank You for your kind comments. My intention is to support students' interest, love and passion in the Sciences of the Universe (particularly Chemistry, and also psychology and philosophy, but these are of course, not examinable at 'O' level), and most importantly, to help students realize that they can (and will) make these Sciences their own. Yes, to OWN Chemistry for yourself. You recognize a scientific phenomenon to be true, not because someone else told you so (ie. dogma), but because you understand within yourself the scientific principles that make such phenomena a natural and beautiful consequence. A consequence of the wonderful Universe, that you belong to, as well as that belongs to you.
Enjoy Chemistry for life, Garrick, whether or not you choose to continue to study it as an academic subject in the tertiary levels (eg. 'A' level, Polytechnic, University, etc).
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No one wants to attempt the Ultimate Stoichiometry Qn? Pity... it would have been a good eye opener for 'O' level students... remember, you only function at your best when pushed to your limit...
Just look at Max Payne... though I believe tragedies (it began with the tragic murder of his wife and daughter) should never be have to be mandatorily required for great men/legends/greatness to happen. That itself would be a tragedy.
But what matters is what you do from whatever situation you're given, whatever life you're born into, whatever karmic situation you find yourself in, at any given time. Ask yourself : What can you do, NOW, to progress/evolve, making everything better for yourself and for the universe?
Here's a little start :