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March 29, 2024, 09:03:22 am

Author Topic: VCE Chemistry Question Thread  (Read 2313419 times)  Share 

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Rod

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Re: VCE Chemistry Question Thread
« Reply #180 on: January 22, 2014, 01:22:15 pm »
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I got around concepts like endpoint, equivalence point, average titre, aliquots, primary standards, standard solution, the types of cuves in pH graphs and all the equipment used during a titration when I did a practical on titration in year 11. So I think it is essential that you ask your teacher to do a practise prac on titrations and even back titrations, it will help a lot.
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #181 on: January 22, 2014, 01:35:13 pm »
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I think I understand now..
I think I was assuming that when the colour changes (permanently), that was the equivalence point...but that's the end point right?
Doing a titration alone can't determine the precise/true equivalence point, but only a close indication of the equivalence point?

In other words, the end point shows us the physical event (colour change), while the true equivalence point is determined based on the end point?

PLS SAY I'M RIGHT
omg. this seems like a straightforward concept, but I seem to be unsure...


EDIT: Just read thushans post again..What he means is what I said in this post, right? :S
« Last Edit: January 22, 2014, 01:38:25 pm by Snorlax »
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Rod

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Re: VCE Chemistry Question Thread
« Reply #182 on: January 22, 2014, 01:43:51 pm »
+1
I think I understand now..
I think I was assuming that when the colour changes (permanently), that was the equivalence point...but that's the end point right?
Doing a titration alone can't determine the precise/true equivalence point, but only a close indication of the equivalence point?

In other words, the end point shows us the physical event (colour change), while the true equivalence point is determined based on the end point?

PLS SAY I'M RIGHT
omg. this seems like a straightforward concept, but I seem to be unsure...


EDIT: Just read thushans post again..What he means is what I said in this post, right? :S
Yes you've nearly got it! Good job!

There is just one thing that you said that I feel isn't correct. The equivalence point NEVER makes the colour change. It's just a specific pH at which the standard and uknown both come to the appropriate stoic proportions. Scientists know at what pH the equivalence point is, so by knowing this we can use an indicator (this makes the colour change) which changes the colour when it is at the correct pH. So for example, it's like we react two solutions that are at correct stoic proportions at a pH of 4 (so the equivalence point is 4). We would then use an indicator that changes colour at a pH close to 4 (EXAMPLE - Methyl orange which ranges from 3 to 4.4). Thus, it will indicate a close approximation of the equivalence point. In other words, the end point is used to determine the equivalence point.

Other then that, everything else you have said is great. Good job bud!

Rod
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Re: VCE Chemistry Question Thread
« Reply #183 on: January 22, 2014, 03:07:45 pm »
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Thanks Rod and Brightsky, should I just remember when I ratio: put the coefficient of the mol of what I am looking for on top of the fraction and what I'm ratio-ing it against at the bottom :P Thanks heaps!!

Could anybody help me out with this question?

Ammonia is produced from its elements on a large scale using the Haber process (what on earth is this..?!). For reasons of environmental safety the concentration of ammonia in the air downwind of an ammonia production plant was mreasured by the following procedure:

a 20000 litre at stp sample of the air was slowly bubbled through an excess of dilute hydrochloric acid. the resulting solution was made alkaline and heated, the ammonia liberated being dissolved in exactly 50ml of 0.10 M HCl acid, which is a large excess. 40.00 mL of 0.10 M sodium hydroxide of 0.10 M Solution were required to neutralise the excess of acid.

Calculate the concentration of ammonia in the air in units of moles of ammonia per litre of air.

Is this some type of back titration?! Could someone give me some hints im so lost.. What do I do with the whole STP thing? What should I be thinking as soon as I read STP or SLC in a question? Thanks

TimewaveZero

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Re: VCE Chemistry Question Thread
« Reply #184 on: January 22, 2014, 05:22:20 pm »
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Just after some advice on how to word a discussion question.

Compare your calculated result with the manufacturers analysis.

The prac was to find by means of redox titration, the concentration of iron in a fertiliser. I calculated it to be 18.58%, with the manufacturers analysis saying 20%. How should I word the question?

Should I just say something along the lines of: The calculated result compares relatively well (does it?) with the concentration of iron supplied on the label by the manufacturer, considering human error in the process of our analysis.
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brightsky

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Re: VCE Chemistry Question Thread
« Reply #185 on: January 22, 2014, 08:15:54 pm »
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Just after some advice on how to word a discussion question.

Compare your calculated result with the manufacturers analysis.

The prac was to find by means of redox titration, the concentration of iron in a fertiliser. I calculated it to be 18.58%, with the manufacturers analysis saying 20%. How should I word the question?

Should I just say something along the lines of: The calculated result compares relatively well (does it?) with the concentration of iron supplied on the label by the manufacturer, considering human error in the process of our analysis.

no, scientists hate vague adverbs like 'relatively', 'fairly', 'rather', etc.. state clearly whether the calculated concentration of iron is higher or lower than the quoted concentration of iron, and give the stats.
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TimewaveZero

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Re: VCE Chemistry Question Thread
« Reply #186 on: January 22, 2014, 08:18:34 pm »
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Oh okay, my Chem teacher loves the word relatively :/ haha

I'll reword it then.
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Jason12

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Re: VCE Chemistry Question Thread
« Reply #187 on: January 23, 2014, 04:57:18 pm »
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When balancing equations does 6HNO3 mean that there are 6 hydrogen, 6 nitrogens and 18 oxygen? I really should know this but I've forgotten
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brightsky

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Re: VCE Chemistry Question Thread
« Reply #188 on: January 23, 2014, 04:59:34 pm »
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indeed. in its pure form, HNO3 exists as a molecule. think of 6HNO3 as six individual HNO3 molecules. if you tally the number of hydrogen, nitrogen and oxygen atoms respectively, you'd get what you got.
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eagles

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Re: VCE Chemistry Question Thread
« Reply #189 on: January 24, 2014, 09:05:06 am »
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Regarding the experimental set-up of evaporation (gravimetric analysis), is it necessary to write where to heat the sample?

For instance, "take the sample and heat in a desiccator until constant mass" or is it better to leave the desiccator bit out?

Thanks!

brightsky

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Re: VCE Chemistry Question Thread
« Reply #190 on: January 24, 2014, 09:13:11 am »
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yeah i've never seen the term 'desiccator' included in the description. you can heat it up in an oven if you really wanted to. just write 'heat sample to constant mass'.
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eagles

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Re: VCE Chemistry Question Thread
« Reply #191 on: January 24, 2014, 09:22:50 am »
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Ok thanks!

T-Infinite

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Re: VCE Chemistry Question Thread
« Reply #192 on: January 24, 2014, 09:57:23 pm »
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Hey guys, quick question. How do you find the amount in mol of the ions, when you're only given the concentration and the molar mass? I may have forgotten some steps. Question is,

For a 0.20M solution of potassium sulfate, K2SO4, calculate the amount, in mol of:
a) potasium ions, K+
b) sulfate ions SO4^2-
c) oxygen atoms

« Last Edit: January 24, 2014, 10:12:30 pm by T-Infinite »
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T-Infinite

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Re: VCE Chemistry Question Thread
« Reply #193 on: January 24, 2014, 10:09:07 pm »
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When you have 0.20M of K2S04, consider the fact that all the K+, SO4 2- etc are all in the same solution i.e they all have the same volume. Now, let's bring back to the mole equation n= C x V. Obviously, if everything has the same volume now, so n = C. Hence, 0.20M of K2S04 will have 0.40M of K+ i.e 0.40 mol of K+ as well. Apply the same procedure for the others :D!
Hope this helps.
How come the answer for a) says it's 0.10mol ? o-o
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nhmn0301

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Re: VCE Chemistry Question Thread
« Reply #194 on: January 24, 2014, 10:18:58 pm »
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How come the answer for a) says it's 0.10mol ? o-o
If it's a textbook question then I'm probably wrong at the finding the mole point. But I'm sure that if you have for e.g. 2M of H2SO4, there will be 4M of H+ if complete dissociation occurs. Sorry if this doesn't help.
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