Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: mass on February 27, 2008, 01:20:50 PM
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thanks.
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I take it cm3= ml?
Are you serious? I suggest you read a basic text on measurement units
http://www.unc.edu/~rowlett/units/ (http://www.unc.edu/~rowlett/units/)
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Please read forum rules (http://www.chemicalforums.com/index.php?page=forumrules). Subject "Help me" doesn't tell anything about your problem.
To be honest - even after reading your post I have no idea what you are asking about. The only answer to only question yous posted is "yes".
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I am asking how to do the calculations the question is asking for?
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Start with reaction equations and definitions of things you have to calculate.
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Unless you start showing some progress this thread will probably get locked (Inote that this is your second thread today on the same subject.
I think Point 4 of the forum rules sums things up nicely.
"4.) Please show that you've at least attempted the problem. We don't mind helping you solve problems but we are ethically opposed to doing homework for you. Violators will have their topic deleted or locked."
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jesus christ, i honestly don't know where to start hnece I am asking.
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A suggestion: read your book again, write down information that's pertinent on a piece of scrap paper, and use it to solve some of the problems. Show us what you have, and what you've written down, and we'll help you with some places you're stuck.
And ditch the scanner -- you're robbing yourself of a chance to write down what you've read to help yourself understand it better. Look at this question: http://www.chemicalforums.com/index.php?topic=23190.msg88389#msg88389
Someone scanned a problem and couldn't solve it, they didn't even realize they hadn't read it correctly.
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I am not trying to get anyone to do my work. At university there is no guidance and hence we are all struggling. Therefore, I am having to use this forum for help. I know the equations relating to partition coefficient but I want to know how on earth you can use them?
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I know the equations relating to partition coefficient but I want to know how on earth you can use them?
Most likely you have to find a way to find concentrations that you should put into the partition coefficient definition... You know initial amount of benzoic acid, this will not change for obvious reasons, although it will be split between two phases. Titration is a way of determining amount of the acid in the water - do you know what reaction takes place?
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Stop and think what you have just measured.
You were given a sample of known concentration of benzoic acid.
It is a week acid therefore it was not fully ionized
You have a titration value for a benzoic acid sample + buffer that was extracted with DCM. What might the DCM remove?
You have a titration value for buffer alone extracted with DCM.
What is the difference between the above titrations, so therefore what can you now calculate.
How does that value compare with the given concentration of the original sample?
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I know the equations relating to partition coefficient but I want to know how on earth you can use them?
Most likely you have to find a way to find concentrations that you should put into the partition coefficient definition... You know initial amount of benzoic acid, this will not change for obvious reasons, although it will be split between two phases. Titration is a way of determining amount of the acid in the water - do you know what reaction takes place?
I am not sure about the reaction taking place. Could I use n=cv/1000 and find the conetration ???
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Neutralization.
Could I use n=cv/1000 and find the conetration ???
If I will answer yes - will you know what you are doing? If I will answer no - will it help you understand anything? Calculation have to reflect stoichiometry of the reaction taking place. As long as you don't know the reaction, you can't use some equation taken out of the thin air and hope it'll be OK.
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no I don't get it still ,whats the equation though? sorry my level of chemistry has become so poor since I have come to uni.
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I believe you were already given one of the reaction "equation" here:
http://www.chemicalforums.com/index.php?topic=23518.0 (http://www.chemicalforums.com/index.php?topic=23518.0)
I suggest you:
1. calculate the [H+] present from the titration data in the "test" section of the problem.
this gives you the following information pH (from your measurement),Pka (listed) and [H+].
Check you textbook (in the chapter on pH) and find an equation which requires all of the above (and contains an unknown quantity).
2. Calculate the value of the unknown.
3. Repeat step 1 for the blank.
This should supply you with all the information you need to answer the questions.
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I set up a student thing yesterday afternoon so me and a few other students could discuss the questions.
We came up with the equation
C6H5COOH + NAOH = C6H5COONA+ + H20
Its a 1;1 ratio. However we were stuck after this stage. We attemepted the % question but we were getting ridiciolous values like 168%. Can someone take me through how to do it please.
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C6H5COOH + NAOH = C6H5COONA+ + H20
Close - although it is... not balanced. For the reaction to be balanced not only atoms must be balanced, but also the charge. In your case there is +1 on the right. (Not to be nitpicky about NA instead of Na).
So the correct version is
C6H5COOH + NaOH -> C6H5COONa + H2O
It gives the same ratio 1:1.
Its a 1;1 ratio. However we were stuck after this stage. We attemepted the % question but we were getting ridiciolous values like 168%. Can someone take me through how to do it please.
Have you not forgot about your blank? Your solutions contains not only benzoic acid, but also phosphoric buffer. This buffer is nothing else but a mixture of phosphoric acid and some base - but acid is in a excess. So when neutralizing your solution you have to neutralize BOTH benzoic acid and phosphoric acid from the buffer. Blank titration should tell you how much phosphoric acid was in your sample - you should substract this amount from the results of the sample titration.
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what question are you helping me with though ???
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Part of your work is to find - from titration - amount of the benzoic acid left in water solution. For now you have not yet calculated this amount properly. I am referring to this particular calculation.
Show how you got to 168%.
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So you are referring to the fraction of benzoic acid unionised in the aqueous phase.
As for getting the 161%, this is my calculation:
C1V1 = C2V2
0.02 V1= 0.051 *31.5
Therefore, v1= (0.051 *31.5)/ (0.02) = 80.325cm3.
Therefore the % of benzoic acid extracted is = 80.325/50*100 = 160.65%. This was question 4.
See I do have a go but I need guidance.
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Problem is, you are ignoring what I am telling you. 31.5 mL (shouldn't it be 31.5-0.5=30?) contains also base that was used to neutralize phosphoric acid, that's what the blank was for. I have already explained that in details.
Not to mention the fact that you are calculating amounts comparing volumes, not numbers of moles. I have no idea where to begin with the help. To help I have to build on some knowledge that you should already have :(
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ok yeah 30 then, my calculation is still right no?
could you not do it and show it me please, i am trying my level best but of to no use. Atleast if you do it for me, I can try and carry on next time I get a similar type of question.
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ok yeah 30 then, my calculation is still right no? No it's wrong, you have ignored and information provided by the blank experiment (for a start)
could you not do it and show it me please, i am trying my level best but of to no use. Atleast if you do it for me, I can try and carry on next time I get a similar type of question.
If we do your work for you, you will learn nothing. If that method of education worked, your lecturers and professors would provide full answers for all the problems you are being set, thus allowing you to learn unhindered by any intellectual challenge. :-\
As this will be my last post on this thread, au revoir