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March 29, 2024, 10:24:27 am

Author Topic: VCE Chemistry Question Thread  (Read 2313466 times)  Share 

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eagles

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Re: VCE Chemistry Question Thread
« Reply #510 on: March 23, 2014, 08:46:58 pm »
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I was just wondering about the reaction of water and carbon dioxide gas which gives us carbonic acid.
Would we use a complete reaction arrow or an equilibrium arrow?

Thank you.

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #511 on: March 23, 2014, 09:24:33 pm »
+3
If you were to draw in arrow sizes to reflect the proportion of completion of the reaction...you wouldn't be able to see much of the forward arrow :P
It's not a very favourable reaction. Carbon dioxide doesn't dissolve in water to any significant extent, nor does it form much of an acid. That's why when we write acid-base equations with carbonate, we neglect the formation of carbonic acid and go straight to H2O and CO2.
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eagles

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Re: VCE Chemistry Question Thread
« Reply #512 on: March 23, 2014, 10:36:44 pm »
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Haha, thanks.

Given a class prac, we need to find the titratable acidity content in wine, as expressed if all is tartaric acid, which is a diprotic acid with molar mass 150g mol-1. I'm having trouble understanding what this means and would really appreciate an explanation.

Also, I'm unsure about the purpose of step 4 in our experiment of determining acidity in wine:

1. Fill the burette with NaOH.
2. Add approx 100mL of deionised water to a 250mL conical flask.
3. Add 3 or 4 drops of phenolphthalein indicator to the flask and mix well.
4. Add 0.1M NaOH from the burette to the flask until the solution turns a pale pink colour that persists for at least 30 seconds. This takes only a few drops and there is no need to record this volume.
5. Pipette 10.0mL of wine into conical flask and record initial burette reading.
6. Titrate the solution in the conical flask with NaOH until the pink colour again persists for at least 30 seconds.
7. Record final burette reading and repeat steps to obtain concordant titres.

Does this mean that the pink colour fades when we do step 5? Otherwise, how are we expected to see the pink colour change as mentioned in step 6? Had we omitted this step, that is pipetted the wine with the deionised water and phenolphthalein (without a drop of NaOH), would it have made any difference?

Can someone please also explain the need for adding the 100mL of water? I understand that it does not change the total amount of acid in the sample, so still unsure why step 2 is necessary.

All of your help is appreciated! Thank you  :D
« Last Edit: March 23, 2014, 10:49:57 pm by eagles »

ETTH96

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Re: VCE Chemistry Question Thread
« Reply #513 on: March 24, 2014, 06:51:52 pm »
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hey guys, sorry this may sound stupid but... in gravimetric analysis where a precipitate is formed, how do you know what to use to react with your substance to form a particular precipitate?

lzxnl

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Re: VCE Chemistry Question Thread
« Reply #514 on: March 24, 2014, 07:00:42 pm »
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Depends on what salt you want. If you want to precipitate silver chloride from silver ions, for instance, you need silver and chloride ions to be present in water. For instance, react silver nitrate with hydrochloric acid to form silver chloride precipitate and nitric acid.
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Re: VCE Chemistry Question Thread
« Reply #515 on: March 24, 2014, 07:02:31 pm »
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Referring to the solubility table is quite helpful.

There are some precipitates which are good to remember that often appear in tests such as BaSO4 and AgCl.

For example, if given a reactant such as AgNO3, and in order to make a precipitate such as AgCl, we need a reactant with Cl in it. We can choose NaCl.

AgNO3 (aq) + NaCl (aq) --> AgCl (s) + NaNO3 (aq)


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Re: VCE Chemistry Question Thread
« Reply #516 on: March 24, 2014, 11:35:50 pm »
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hi everyone I am new to this site and finally received my Extended Experimental Investigation: Analysis of fertiliser sac results back for which I only got 24/50 and after asking my teacher for reasons as to why I got this result he could not explain, would someone please help me out and post a previous sac which received the same or higher mark so I can see where I went wrong before I make the same mistakes on the next practical sac. I really want a good mark chemistry please help me out.

if anyone is on please help me out I'm that stressed I don't know what to do any more.  :'(
« Last Edit: March 25, 2014, 02:05:38 pm by shaw »

ETTH96

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Re: VCE Chemistry Question Thread
« Reply #517 on: March 25, 2014, 06:00:48 pm »
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Hey guys, I have a chemistry SAC tomorrow and it is based on the analysis of fertiliser; the sulfur, nitrogen and phosphorus contents. part of the SAC is that we have to draw up 1 table summarising all our results. The three methods of analysis we used were; gravimetric, volumetric(back titration) and colorimetric... so how can i go about setting up a table for my results? because i have no idea :(

mikehepro

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Re: VCE Chemistry Question Thread
« Reply #518 on: March 25, 2014, 07:19:49 pm »
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Hey guys, just wondering how should I approach this question?
of a compound containing carbon,hydrogen and oxygen produced of water and of carbon dioxide on being burnt at s.t.p. Calculate the empirical formula of the compound.
I got c:8 H:24 O:7 but that looks wrong to me.
Any help would be greatly appreciated.
Thanks.
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Phenomenol

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Re: VCE Chemistry Question Thread
« Reply #519 on: March 25, 2014, 07:26:52 pm »
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Hey guys, just wondering how should I approach this question?
of a compound containing carbon,hydrogen and oxygen produced of water and of carbon dioxide on being burnt at s.t.p. Calculate the empirical formula of the compound.
I got c:8 H:24 O:7 but that looks wrong to me.
Any help would be greatly appreciated.
Thanks.

Hmm, try again. Remember, to find the mass of oxygen originally in the organic compound, subtract the mass of carbon and mass of hydrogen from 0.470g.
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Snorlax

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Re: VCE Chemistry Question Thread
« Reply #520 on: March 25, 2014, 07:48:40 pm »
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An alcoholic beer has a conc. of 2.5% v/v of ethanol. (density of ethanol @ 0.5g/mL)
Find the amount of ethanol consumed in a 375mL can. Mr of ethanol=46

Thanks
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Phenomenol

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Re: VCE Chemistry Question Thread
« Reply #521 on: March 25, 2014, 08:12:05 pm »
+1
An alcoholic beer has a conc. of 2.5% v/v of ethanol. (density of ethanol @ 0.5g/mL)
Find the amount of ethanol consumed in a 375mL can. Mr of ethanol=46

Thanks

Concentration m/v = concentration v/v * density of solute
Mass = volume * concentration m/v
Amount = mass/molar mass

Make sure your units are consistent as always!
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Re: VCE Chemistry Question Thread
« Reply #522 on: March 25, 2014, 08:28:19 pm »
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Hmm, try again. Remember, to find the mass of oxygen originally in the organic compound, subtract the mass of carbon and mass of hydrogen from 0.470g.

Right,so I found out the how many mols of Co2 and H2O there is, which is roughly 0.03 and 0.02, however, how am I suppose to determine the mole ratio? Since there's o2 on the left side of the equation, unless I'm not doing it with the right method in the first place.
Thanks
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Phenomenol

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Re: VCE Chemistry Question Thread
« Reply #523 on: March 25, 2014, 08:35:47 pm »
+1
Right,so I found out the how many mols of Co2 and H2O there is, which is roughly 0.03 and 0.02, however, how am I suppose to determine the mole ratio? Since there's o2 on the left side of the equation, unless I'm not doing it with the right method in the first place.
Thanks

The trick here is to look at the masses of elements first, not amounts. Otherwise you're doing too much work.
Total mass(H) = 2.0/18.0 * 0.551g. This gives you the mass of hydrogen in the organic compound.

Total amount(CO2) = 0.458/22.4
Then find the mass of carbon in the CO2. This gives you the mass of carbon in the organic compound.

Simply subtract these masses from 0.470g to find the mass of oxygen in the compound. From there the routine method to find an empirical formula can be used.

So, the important thing to note is that all the carbon in the original compound is now in the form of CO2, and all the hydrogen in the original compound is now in the form of H2O.
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Re: VCE Chemistry Question Thread
« Reply #524 on: March 25, 2014, 08:46:31 pm »
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There's a difference between de-ionised water and destilled water..But are they the same? Just difference processes?
If a question asked to write steps for like...cleaning equipment..which would I use?

Thanks
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