Chemistry 3/4 2013 Thread

[Reckoner]

Sorry about the bad phrasing.

What I wanted to ask was why is the gradient steep in some places and gentle in other places. Or why isn't the change in pH linear.

Thanks.

I'm fairly sure its to do with pH being a log, but we are looking at a graph with a linear scale. http://www.scienceforums.net/topic/15586-ph-titration-curves/ See the third post.

So I've got a question related to a titration between iron ions (fe2+) and pottasium permanganate.
One of the questions in the prac book (which for the life of me I can't think of an adequate answer) is:
5. The amount of additional potassium permanganate solution required to reach the endpoint is minute. This excess will notionally increase the calculated amount of iron present but, in practice, the extra has no effect on the calculated amount. Why not?
I'm really stumped on this one.
Also, another question asks: In most titration exercises, the titres are averaged before doing the calculations. Why is this
inappropriate in this exercise?
Thanks for helping out!

For the first part, we can't know how many drops over the equivalence point we have gone to reach the endpoint (if we did, why not just use the equivalence point titre, rather than the end point!), so we cannot accurately account for the few drops in our calculations. In turn, we have to assume that the endpoint equals the equivalence point, otherwise we can't do our calculations! This is why choosing an appropriate indicator to decrease this difference is essential.

For the second without seeing the results, I would assume that they are not concordant.

[Bad Student]

Thanks. That's exactly what I was asking for.

[Jaswinder]

Hey guys,

With chromatography how can you be sure that the peak in the sample was your intended analyte and not any thing else with a close or similar retention time?

[thushan]

Hey guys,

With chromatography how can you be sure that the peak in the sample was your intended analyte and not any thing else with a close or similar retention time?

You can't. That's one of the limitations of chromatography; it can't distinguish between two compounds with the same retention time. If the two compounds had similar retention times, but not the same, you can finely distinguish between the two. However, if you can't differentiate between the two compounds, one way of getting around that is to run the two under a different fluent.

[Jaswinder]

If the two compounds had similar retention times, but not the same, you can finely distinguish between the two.

how would you distinguish between them?

[thushan]

If the chromatography were high enough quality (for instance GC or HPLC), you can treat a retention time of say 96 seconds and a retention time of 97 seconds as of two different compounds.

[Jaswinder]

yeap this was actually an practical, we had to find out the concentration of caffeine using hplc. however when we did the sac we didnt get the exact time but we knew it was caffeine, coz there were no other peaks lol :/

so just the variation in the retention time would distinguish the samples yeh?

[thushan]

yeap this was actually an practical, we had to find out the concentration of caffeine using hplc. however when we did the sac we didnt get the exact time but we knew it was caffeine, coz there were no other peaks lol :/

so just the variation in the retention time would distinguish the samples yeh?

Mm. Could be due to a variety of factors. Flow rate (how fast is the solvent being pushed through the column?), temperature, the stationary phase, these can all affect retention time.

[saba.ay]

Heinmann

A student titrated an aliquot of standard sodium carbonate solution with HCl acid in a burette. State whether the concentration determined for the HCl would be likely to be higher than, lower than or unchanged from the actual value if the student had previously washed with water, but not dried, the following apparatus:
a) the pipette used to deliver the aliquot of the sodium carbonate solution
b) the flask containing the aliquot
c) the burette

Please and thank you  

[Daenerys Targaryen]

Higher- if water is left in pipette with NaCO3 then it becomes more dilute (less NaCO3 in the pipette becuase water is taking its spot), thus the HCl will be higher than that of NaCO3
Unchanged
Lower

[Bad Student]

I can't get the right answer for part f of the question. In part e, my answer is 0.0162 mol which is correct according to the answers.

To work out the mass of iron sulfate in part f,

0.0162 x (55.8 + 32 + 4(16)) = 2.50 g

but the answer in the textbook is 0.246g. Where have I gone wrong?

[teletubbies_95]

The question is asking for each tablet!

[Bad Student]

The question is asking for each tablet!

THANK YOU!!! I should read the question next time.

[Mr Keshy]

Living up to your name eh mate?


I kid I kid

[Mr Keshy]

How are you all finding chem so far? I'm finding that I understand it really well. I love it. But methods put me behind in chem class a little so I'm playing a little catch up!

I trust all you guys are smashing through