Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Aetos on November 03, 2011, 01:23:20 PM
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PLEASE SEE THE ATTACHMENT ALSO
Standard Flask Concentration Absorbance(254nm)
No. mM
1 0.04 1.2
2 0.5
3 0.2
4 0.08
5 0.02
Working out the concentration:
C1 V1 = C2 V2
(C1 V1)/V2 = C2
Standard flask 1:
C1= 0.1M
V1= 10ml
V2= 25ml
(0.1x 10) / 25 =0.04mM
standard flask 2:
what is the V1 value for flask 2? How do I work it out?
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can't anyone help me ???
trying to find:
what the V1 value is for flask 2? and How I work this out?
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Umm ... can't you see that you just plug in values, just like you did to determine the answer to part 1? If you try writing out the important parts of the question, instead of scanning and posting the image, you might be able to figure it out on your own. Try it and see.
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I DID NOT scan and just post the image. I tried understanding the question myself and as you can see I showed some working out. The reason I am confused about the working out for flask 2 is because I dont know what the initial volume is, because it doesnt say. I am assuming the initial volume decreases, therefor becomes more diluted, but how to i know what the initial volume for the rest of the flasks?
I am sorry if it seems like a stupid question for all you 'smart' people out there but I am new to this kind of chemistry stuff.
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I DID NOT scan and just post the image.
Yes. Yes you did. Or at least, you've posted an image. Maybe you got it from somewhere else, but yes, you've skipped writing it out.
I tried understanding the question myself and as you can see I showed some working out.
Awesome. Now, do it again.
The reason I am confused about the working out for flask 2 is because I dont know what the initial volume is, because it doesnt say.
Actually, the image you posted does say. If it were text that you'd had typed, I'd have highlighted it for you.
I am assuming the initial volume decreases, therefor becomes more diluted, but how to i know what the initial volume for the rest of the flasks?
Umm... no, that's not true, in this case.
I am sorry if it seems like a stupid question for all you 'smart' people out there but I am new to this kind of chemistry stuff.
I know, I've read your other posts. Try this one again and see if you can crack it. Start from the beginning, and redo the first part that you did. Try it out on paper.
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I am assuming the initial volume decreases, therefor becomes more diluted, but how to i know what the initial volume for the rest of the flasks?
Umm... no, that's not true, in this case.
well you are wrong! if YOU READ the text like I DID then you would notice that it says that it is a 'serial dilution'. It also says that the 10 ml sample from standard 1 is used to make standard 2, and 10ml from standard 2 is used to make standard 3 and so on. So obviously it would become more diluted as there would be less stock solution from flask to flask. So what would be the initial volume? i doubt it would be 10ml again because I did a similar experiment where the initial volume was different for each flask as it became more diluted. ALL I NEED TO KNOW IS HOW TO WORK OUT THE INITIAL VOLUME FOR EACH FLASK. IF IT IS 10ML THEN JUST SAY SO!
If you're not going to help me then FINE! do not reply to this topic, simple as!
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Calm down. Arckon IS helping you. Wording of the question is clear, perhaps you just have to take a break and read it again later.
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??? instead of telling me to read it again later will someone just tell me how to work out the initial volume for flask 2? It cant be 10ml because then the values for concentration would be the same for all flasks which isnt the case. give me the formula, w/e just give me an idea of how to work it out. im not asking you to do my homework for me just to help me. Telling me to read it again isnt helping me because I am still stuck in the same place with the same questions!
And the reason I cant be calm is because this is only the first part of my home work and i still have to draw a graph and make conclusions and this is due next week.
Again if you cant help me then DO NOT post, stop wasteing my time.
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okay all i need to know if this is the right answer/ method for working out the concentration for flask 2. 'right' or 'wrong' response will do.
10ml of stock in 15ml of phosphate buffer solution.
Percentage of stock in flask 1:
10/25 x 100= 40%
10ml of flask 1 is transferred into flask 2
40% of 10ml = 4ml
Which means there is 4ml of stock in tube 2.
C1= 0.1M
V1= 4ml
V2= 25ml
(0.1x 4) / 25 =0.016mM
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Now that you started to show your work it is clear where the problem lies. You tried to use rather convoluted way of calculating concentration, so we were not able to decipher your question.
While you can try to calculate concentration in serial dilution in terms of "volume of the initial solution" passed, that's not how it is usually done. You calculate new concentration after dilution, and use this new concentration together with volume of the diluted solution taken (10 mL in this case) to calculate number of moles of substance passed to the next step. It is amount of substance that is important, not "volume of the initial solution" - even if these are related.
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oh i see so the C2 value for flask 1 is the C1 value for flask 2? and the initial volume of 10ml and the final volume of 25ml stays the same?
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so is this correct?
standard flask 2:
C1= 0.04M
V1= 10ml
V2= 25ml
(0.04x 10) / 25 =0.016mM
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??? i got the exact same results using this method. so either way is correct?
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i got the exact same results using this method. so either way is correct?
They both should yield the same result, as they are mathematically equivalent. I never stated it is wrong - just unusual.
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oh i see so the C2 value for flask 1 is the C1 value for flask 2? and the initial volume of 10ml and the final volume of 25ml stays the same?
Yes, that's the whole idea behind serial dilution. You make them in series, one after another, using the last one to make the next one, so you end up with a linear series that makes for a good way to bracket an unknown sample. You can make a group of varying concentrations from scratch individually, but this way insures a linear range. You can make them using identical aliquot into identical vessels, as in this case. Or you might make them with different volumes. But what really drives the series is that the concentration C1 changes each time.
Here's a tip, do this math using a spreadsheet program. Once you have a table of values, you can work backward, which may come in handy when you have experimental values associated with this dilution series.
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oh right well i get it now
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Hi i have to do this too, basically i worked out the concentration using C1V1=C2V2
The experiment that was conducted was to calculate approximately the amount of drug in a blood serum sample.
A stock solution (100mM) of the drug that was being investigated in a 0.1M phosphate buffer (pH 7.2) was used to set up 5 standard solutions in 25ml volumetric flasks by serial dilution.
An aliquot of stock solution of 10ml was placed in the first volumetric flask and was then made up to the mark by adding phosphate buffer solution to the flask; this mixture was thoroughly mixed and labelled standard 1. Then An aliquot of standard 1 10ml was then taken and put into the second 25ml volumetric and made up to the mark and the second flask labelled standard 2. This process was repeated, 10ml of standard 2 was used to make standard 3,10ml of standard 3 used to make standard 4 and finally 10ml of standard 4 was used to make standard 5.
so what i did was Standard 1
C1= 0.l 0.1x10/25 = 0.04 mM
V1= 10
C2=?
V2= 25
and so on, but im unsure now whther this is right,because should i have multiplied 100x10/25 because 100mM is the stock solution concentration
CAN ANYONE HELP PLZ
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*looks up*
who are you? you have the same H/W questions as me! =D