AQA A2 Planning exercise practical... Watch

croakly
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Hey the only hard thing about this practical is the planning because the implementing is either titration or observation and analysis is okay.
However the planning can be either hard or easy.

I have revised how to plan:
pH
Titration
Enthalpy ( unlikely... AS standards)
Aspirin recrystilation
Maybe Friedels and nitration
Prepartion of standard solution

Does anyone else know what else they can ask... or they can ask anything?

Can the electrode potentials come up? Like plan how you find e.m.f and how to set the stuff up etc.
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happyfacedani
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The preparation of a standard solution was in this years AS paper in the planning section so prob wont come up in the A2 as well... I thought maybe something to do with preparing a buffer solution
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croakly
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Lol how do you prepare a buffer?

I never done it in class before hmmm...

I think it has to have something weak i.e weak acid...

Do you make one just like making a standard solution...
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happyfacedani
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I found some practice questions in a revision book, such as how can you make a buffer of pH 4.5 from propanoic acid, pKa = 4.87?
For any weak acid, HA + H2O (equilibrium sign) H3O+ + A-
so apply the equilibrium law to get Ka = [H3O+] x [A-]/[HA]
Then work out Ka from pKa and substitute the values you know into the equation
This gives the ratio of acid to the propanoate you would add, so then you just need to use basic calculations to work out the mass of propanoate to add!
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rustynator
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I am guessing it is either gonna be an organic plan (e.g. preparing x, then recrystallising it), a plan to determine the Ka of an acid (pH measurement etc.) or a good old titration plan.
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croakly
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(Original post by happyfacedani)
I found some practice questions in a revision book, such as how can you make a buffer of pH 4.5 from propanoic acid, pKa = 4.87?
For any weak acid, HA + H2O (equilibrium sign) H3O+ + A-
so apply the equilibrium law to get Ka = [H3O+] x [A-]/[HA]
Then work out Ka from pKa and substitute the values you know into the equation
This gives the ratio of acid to the propanoate you would add, so then you just need to use basic calculations to work out the mass of propanoate to add!
okay...

so you would assume [H+] = [A-] therefore Ka = [H+]2 / [HA]

to get Ka ... pKa = -logKa therefore Ka = 10-pKa

What figures do we have [H+] or [HA]? Then rearrange that to get one of those?

I dont understand how you know how much you add to get the mass of propanoate... I know the equations but I think you will need to give a numerical example... thanks
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Featherflare
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Well lets say u had:

HA + H2O <----> H3O+ + A-

if the moles of HA was initially 1 mol. then u worked out the moles of A- to be 0.2 in equilibrium. Then the moles of HA in equilibrium would be 1-0.2= 0.8

So the ratio would be 0.8:0.2 in equilibrium. Each mole of HA gives 0.2 moles of A-

You can then use this simple ratio to work out the mass needed using mass= n*Mr
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happyfacedani
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So how did people find it?
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lufiaguy
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Didn't like the plan that much. I spent ages revising the pH and buffer stuff, so a little annoyed that they weren't on it at all!
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Featherflare
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It was ok. The last question in the evaluation was iffy but i got at least one of the reasons right. The implementing was a joke.. just adding A and B and saying what colour it turns, my 3 yr old niece culd do tht. The plan was a little tricky but it was quite doable once i thought it thru
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moomoo2k
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what were the things for the evaluation?i had no idea
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rustynator
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I put
1)The stuff decomposes therefore some will be lost during the experiment
2)Some solution will be lost during the transfer from pipette to volumetric flask

In the implementing part how did everyone describe the cloudy solutions....I read in the mark scheme that you are meant to use the word "cloudy", so instead I put the word "precipitate". Is this correct?
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Featherflare
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precipitate (ppt) is the correct term where relevant
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moomoo2k
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i put the decomposition thing and i also said that there is human error in getting the colour change...i still dont think either would account for the 30% difference or whatever it was!
and yea, precipitate...i hate those observations, you can so easily get it wrong
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rustynator
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Darn right...they are awful if you are colourblind like me!

I didnt think about the colour change either, that is probably what they were looking for.

What did everyone write for their plan?
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happyfacedani
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OK I got way more than 30% for the difference...uhoh
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croakly
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everything was fine except the plan... i didnt expect rate would come up as my teacher never went through it with the class... so i didnt put much effort revising it.

i knew how to do most of everything else eg like standard electro potentials, ph, preparing aspirin, recrystalisation, titration, enthalpy, preparing organic things like by acylation and nitration and an as practical using a syringe... and i even revised buffers.

but not rate... omg... im just annoyed that i revised the above and knew it and they gave me something i didnt know much. now i must get like very good marks on my unit 4 and 5 and the synoptic multiple choice.

i didnt really say much... like choosing 100cm3 to collect and finding the moles...
for the results all i can think of to get first order is plot the results on a graph and if its first order the gradient should be close to 1 but i dunno if this is right.
all the people who study physics with chemistry prob thought this was the easiest planning...

i hate it when boards put something unpredictable... i prefer ocr much better however aqa is good because it gives past papers on their site while ocr doesnt.
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happyfacedani
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Does anyone else remember what value they got for the difference between the student's concentration and the supplier's?
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sunday
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(Original post by happyfacedani)
Does anyone else remember what value they got for the difference between the student's concentration and the supplier's?
0.2 something, maybe? the percentage diff I got was 31.8 i think
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lufiaguy
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Yeah, I got a percentage difference of that sort of magnitude too. Good to know people got the same!
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