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Author Topic: VCE Chemistry Question Thread  (Read 2313133 times)  Share 

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Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8670 on: August 13, 2020, 08:57:43 am »
0
I’m doing the 2010 exam and I know it’s from an old study design but it’s asking me to calculate pH something which we learnt in unit 1/2 Chem......will this be necessary to know as it has been taught before?
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keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8671 on: August 13, 2020, 09:19:43 am »
+6
I’m doing the 2010 exam and I know it’s from an old study design but it’s asking me to calculate pH something which we learnt in unit 1/2 Chem......will this be necessary to know as it has been taught before?

No - it was in the exam then because it was in the old 3/4 study design.

Hello
would this be right

I have no idea why some of those are dots and some of those are Xs - man VCE teaches weird things. Otherwise, all of it's right except for your bond angle for COCl2 - think about it, if you have three things in a circle, and they're equally spaced apart*, what angle would you expect them to make?

*=okay, so the angle between the chloride and oxygen bonds will be slightly different to the angle between the two chlorides, but they'll be approximately the same, and that should be good enough for VCE. See your teacher, since they're the ones that'll be marking this stuff.

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8672 on: August 17, 2020, 02:23:30 pm »
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would the bond angle be 120

also for this question is it calcium carbonate

1729

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Re: VCE Chemistry Question Thread
« Reply #8673 on: August 17, 2020, 03:27:19 pm »
+3
would the bond angle be 120

also for this question is it calcium carbonate
Sorry, I think I deleted my post accidentally  :-\

The milky white indicates a precipitate, and precipitates are typically solid substances. I reckon it would be calciuk carbinate, and all the bond angles in calcium carbonate are 120 because there are 3 electron domains around the central carbon.

Also for these types of problems you have to know a lot of the common ions bond angles, or how to draw Lewis structures.

I don't know why they made this into a question it literally tells you in the problem.

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8674 on: August 17, 2020, 06:55:32 pm »
+6
Sorry, I think I deleted my post accidentally  :-\

The milky white indicates a precipitate, and precipitates are typically solid substances. I reckon it would be calciuk carbinate, and all the bond angles in calcium carbonate are 120 because there are 3 electron domains around the central carbon.

Also for these types of problems you have to know a lot of the common ions bond angles, or how to draw Lewis structures.

I don't know why they made this into a question it literally tells you in the problem.

Bond angle bit was in reference to something I asked them in a previous post ;) To which the answer is yes - the bond angles in COCl2 are, indeed, 120 degrees.

As for why they made this a question - the question is interested in the students ability to take observations and equations use them to make an informed assessment of what just happened. It's simple, sure, but you have to start somewhere - and even some of my first year uni students struggle with this. Either way, the question is worded weirdly - it's unclear if what they're asking is, "what is the milky white substance that appears", or if they're asking, "what turns the limewater into the milky white substance". I would've personally guessed it being the second one, to which the answer would be the carbonic acid formed in situ (that is, within the solution) by carbon dioxide gas and water. Since it's VCE, you'd likely get the marks if you only wrote "carbonic acid, H2CO3". But I guess consult the answers/your teacher/whoever wrote the question in the first place, just to be sure.

Coolgalbornin03Lo

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Re: VCE Chemistry Question Thread
« Reply #8675 on: August 18, 2020, 09:19:55 pm »
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What is the difference between a lipid molecule sources from a plant or from an animal
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Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8676 on: August 18, 2020, 09:35:15 pm »
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For this is the average titre 25.35 and do you not include the second one as you only include concordant values

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Re: VCE Chemistry Question Thread
« Reply #8677 on: August 19, 2020, 10:49:43 am »
+4
What is the difference between a lipid molecule sources from a plant or from an animal
Saturated fat is in animals, unsaturated fat is in plants, saturated fats are usually solid at room temp while unsaturated are liquid.

Basically saturated means each carbon has maximum amount of hydrogens attached, unsaturated means it cannot attain the maximum possible amt of hydrogen attached to each carbon, so indicates a presense of double bond.
For this is the average titre 25.35 and do you not include the second one as you only include concordant values
Why do you exclude a value other than the rough estimate?

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8678 on: August 19, 2020, 06:57:40 pm »
+6
For this is the average titre 25.35 and do you not include the second one as you only include concordant values

I mean, there's not really a hard and fast rule for this... I'd be asking your teacher what they'd accept. My personal thought process is that it depends on what actually happened in the experiment. If your rough titration CLEARLY went past the end point, then why would you include it? It's clearly an experiment gone wrong, it makes no sense. It's almost as if you were doing a series of paintings, then decided instead of including one of the finished ones, you'd use the rough sketch where his ears were the wrong size, he had two left feet, three fingers (total), and one eye, because "I drew his body better, there". Even though the painting is incomplete and necessarily bad in all other respects.

EDIT: oops, I totally misread your post and thought the initial volume was the average titre. If results are concordant, that means that they match within uncertainty. Even though the second titre is different by 0.05 mL, that should be within the uncertainty of the burette, and so you would still use it to calculate the average titre - it's still a concordant result.

Why do you exclude a value other than the rough estimate?

See above. What if one of your other three points went way past the end point?

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8679 on: August 20, 2020, 06:52:07 pm »
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would this be right

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8680 on: August 20, 2020, 06:54:13 pm »
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would this be right

Good question - why did you select those answers for each?

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8681 on: August 20, 2020, 07:15:58 pm »
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the pipette needs to be rinsed with the solution it is going to be filled with , the burette is rinsed with the solution and distilled water and the conical flask is rinsed with distilled water?

keltingmeith

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Re: VCE Chemistry Question Thread
« Reply #8682 on: August 20, 2020, 08:20:51 pm »
+7
the pipette needs to be rinsed with the solution it is going to be filled with , the burette is rinsed with the solution and distilled water and the conical flask is rinsed with distilled water?

This is a very good rote memorisation of material, but it doesn't attack the heart of the question - which glassware should you rinse in which way to preserve the concentration or the number of moles? Firstly to do this, we need to answer what the role of each piece of glassware is:

a) The burette. It must deliver a compound of exact concentration.
b) The pipette. It must deliver a compound of exact concentration.
c) The conical flask. It must hold a compound of exact quantity.

... but why? Well, let's go back to first principles. We want to know the concentration of some acid or base X. We're going to do that by reacting X with a base or acid, Y. That means we're going to have to use the equation:

X + Y ----> some salt + H2O

Knowing that, this means that wherever the reaction takes place, it is VITAL that the moles in that place don't change. Because if they do, then we're going to have an issue. So, whatever we rinse the conical flask with, we have to make sure that what we've rinsed with DOES NOT change the amount of moles of the flask. This also means that whatever we deliver the acid or base with (the pipette or burette) must deliver the same amount of mole each time.

But the problem with the burette and pipette is we aren't delivering an amount of mole - we're delivering a VOLUME. Okay, so how do we compare the volume to mole? By the equation:



From this, you can see quite easily - if volume is going to stay the same EVERY TIME, then we also need the CONCENTRATION to stay the same EVERY TIME, or the mole will change. That means that whatever we rinse the burette and pipette with, they MUST make it so the concentration being delivered doesn't change, or else that will mean that the amount of mole being delivered will change.



Okay, so with this, it's important to know that WHATEVER you rinse the glassware with, then droplets of that is going to be leftover in the glassware. So, let's work with the following example: We are going to be titrating hydrochloric acid against 10 mL of 0.1M sodium hydroxide

The burette

So first we need to identify what is going INTO the burette. In this case, the known volume is the sodium hydroxide, so that means the HCl will be going into the burette. Remember - the burette is always the one with the unknown volume. So, let's consider what happens if you rinse with each material:

  • If you rinse with sodium hydroxide, then there will be droplets of base left in the burette. If you then add acid to the burette, the base droplets will change the concentration of the acid, so it is no longer exact. Remembering from above - if the concentration is different, but the volume stays the same, then the amount of mole delivered will be different - which is what we want to avoid.
  • If you rinse with hydrochloric acid, then there will be droplets of acid left in the burette. If you then add acid to the burette, the droplets will combine - but the concentration won't change, because the concentration of the acid and the concentration of those droplets is the same. The concentration is still exact. Remembering from above - if the concentration is different, but the volume stays the same, then the amount of mole delivered will be different - which is what we want to avoid.
  • If you rinse with DI water, then there will be droplets of water left in the burette. If you then acid to the burette, the droplets will dilute the concentration of hydrochloric acid, because the concentration of acid in the droplets is different to the concentration of acid in the solution. The concentration is no longer exact.

Let's go through this thought process for the other pieces of glassware:

The pipette

So first we need to identify what is going INTO the pipette. In this case, the known volume is the sodium hydroxide, so that must be what the pipette delivers. Remember - the pipette is always the one with the known volume. So, let's consider what happens if you rinse with each material:

  • If you rinse with sodium hydroxide, then there will be droplets of base left in the pipette. If you then add base to the burette, the base droplets will mix with the solution - but because the concentration of them both is the same, again, the concentration remains exact.
  • If you rinse with hydrochloric acid, then there will be droplets of acid left in the pipette. If you then add base to the burette, the acid droplets will neutralise part of the base solution. The concentration will no longer be exact.
  • If you rinse with DI water, then there will be droplets of water left in the pipette. If you then base to the burette, the droplets will dilute the concentration of sodium hydroxide, because the concentration of base in the droplets is different to the concentration of base in the solution. The concentration is no longer exact.

The conical flask

So first we need to identify what is going INTO the conical. In this case, the known volume is the sodium hydroxide, so that means the sodium hydroxide will be going into the conical. Remember - the conical always initally holds what was in the pipette. So, let's consider what happens if you rinse with each material:

  • If you rinse with sodium hydroxide, then there will be droplets of base left in the conical. If you then add base from the pipette to the conical, the base droplets will add extra moles to the conical. The quantity is no longer exact.
  • If you rinse with hydrochloric acid, then there will be droplets of acid left in the conical. If you then add base from the pipette to the conical, the droplets will combine, and some of the moles of the acid will react with the base, reducing the quantity of base that the pipette delivered. The quantity is no longer exact.
  • If you rinse with DI water, then there will be droplets of water left in the conical. If you then base from the pipette to the conical, then the water will dilute the amount of base of added - but the number of moles in the conical will be the same. The quantity is still exact.

Does knowing all of this change the answer you initially gave?

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8683 on: August 20, 2020, 08:47:48 pm »
+1
Yes
The burette- HCL
Conical flask- deionised water
Pipette- potassium carbonate

Chocolatepistachio

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Re: VCE Chemistry Question Thread
« Reply #8684 on: August 21, 2020, 10:08:03 pm »
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Hello
would this be right ,sorry for asking i wasnt there when my teacher went over this

Also how do you know if the molecule is polar or non polar i am quite confused

and thank you for your really detailed explanation above!