VCE Chemistry Question Thread

[HopefulLawStudent]

Hey quick (+ dumb) question but for AAS, why do we need to dilute the analyte? I can vaguely remember this was a question that tended to pop up for AAS but infuriatingly, I never wrote down the answer and I can't remember it rn.

[keltingmeith]

Hey quick (+ dumb) question but for AAS, why do we need to dilute the analyte? I can vaguely remember this was a question that tended to pop up for AAS but infuriatingly, I never wrote down the answer and I can't remember it rn.

... In what situation? Because it's certainly not necessary to dilute an analyte. Do you have the question on you that asked this?

[The Usual Student]

... In what situation? Because it's certainly not necessary to dilute an analyte. Do you have the question on you that asked this?

I think it had something to do with making the spectrum more accurate, as the analyte with the higher concentration was less accurate .... maybe I am getting confused but it was something like that
Something to do with making the results more accurate by restricting the spectrum so the graph did not extend too far

[keltingmeith]

I think it had something to do with making the spectrum more accurate, as the analyte with the higher concentration was less accurate .... maybe I am getting confused but it was something like that
Something to do with making the results more accurate by restricting the spectrum so the graph did not extend too far

That... Sounds silly and not chemically accurate. O.o Idk, maybe there's an analytical chemist out there that can explain better than little ol' me

[The Usual Student]

That... Sounds silly and not chemically accurate. O.o Idk, maybe there's an analytical chemist out there that can explain better than little ol' me

I remember there was a real reason and i went through it in class but I properly did not express it properly.
Something to do with "interpolation and extrapolation" but i need to go through my notes when i get home then i could probs remember it.

[HopefulLawStudent]

... In what situation? Because it's certainly not necessary to dilute an analyte. Do you have the question on you that asked this?

Unfortunately no, I don't have a situation.

We're currently doing some Unit 3 revision and we're going through a pack that the teacher gave us and legit, the question went something like this:

Why would it be necessary to dilute the analyte in AAS?

And I sat there and was like "I know this tends to pop up as a question" but I can't remember the answer for it. 

[HasibA]

^ doesn't it have something to do with calibration graphs or something and if we can't read them , we dilute it we we need a higher value or something (idk sorry guys I tried xD )

[jyce]

Hey quick (+ dumb) question but for AAS, why do we need to dilute the analyte? I can vaguely remember this was a question that tended to pop up for AAS but infuriatingly, I never wrote down the answer and I can't remember it rn.

So, the answer is essentially what you yourself have touched upon and what HasibA has suggested. But let summarise it for you.

In AAS, the analyte may need to be diluted to ensure that its absorbance fits within the range of the calibration graph. Otherwise, you would need to extrapolate (i.e. extend your graph beyond your data points, by continuing the trend), but this may be inaccurate.

[The Usual Student]

So, the answer is essentially what you yourself have touched upon and what HasibA has suggested. But let summarise it for you.

In AAS, the analyte may need to be diluted to ensure that its absorbance fits within the range of the calibration graph. Otherwise, you would need to extrapolate (i.e. extend your graph beyond your data points, by continuing the trend), but this may be inaccurate.

YAAASSS that was it!!!!
is the extrapolate thing a key word? my teacher said it wasn't necessary since it was more of a mathematical concept but does vcaa expect us to know about interpolation and extrapolation for Calibration graphs?

[jyce]

YAAASSS that was it!!!!
is the extrapolate thing a key word? my teacher said it wasn't necessary since it was more of a mathematical concept but does vcaa expect us to know about interpolation and extrapolation for Calibration graphs?

No, you don't need to know the terms extrapolation and interpolation. It wouldn't be a bad idea to mention extrapolation, though. Don't concern yourself with interpolation at all.

[keltingmeith]

o LOL so it actually isn't an AAS thing, then. That holds for any machine.

[strawberry7898]

Can someone explain to me why adding an inert gas does not affect equilibrium? I can see that doing so would increase the overall pressure, at the same time it would also decrease the number of collisions between the reactants because the reactants would now be colliding with the gas instead of each other. The answer that the teacher gave me was that the overall pressure is affected but not the partial pressure. Another teacher said that even though collisions increase, successful collisions do not increase, which I think is what I said earlier. So what's the actual explanation for this? Any help is appreciated 

[Jakeybaby]

Can someone explain to me why adding an inert gas does not affect equilibrium? I can see that doing so would increase the overall pressure, at the same time it would also decrease the number of collisions between the reactants because the reactants would now be colliding with the gas instead of each other. The answer that the teacher gave me was that the overall pressure is affected but not the partial pressure. Another teacher said that even though collisions increase, successful collisions do not increase, which I think is what I said earlier. So what's the actual explanation for this? Any help is appreciated 

It all depends on how the inert/noble gas is added:

 If it is added at a constant volume, the total pressure present in the system will obviously increase. But the concentration of the products and reactants will not change, hence, the equilibrium will not be affected.

If the inert gas is added at a constant pressure, the total volume will increase, therefore, as the number of moles per unit of volume will decrease, as now the moles are more spread out in the system.
Take the following equation for example:
2 NH_{3 (g)} ⇌ N_{2 (g)} + 3H_{2 (g)}
According to Le Chatelier's Principle, as there is now a lower concentration, the system will shift in the direction of more moles (so that the system can offset the lower in concentration), this means that the equilibrium will shift to the right. 

[jyce]

o LOL so it actually isn't an AAS thing, then. That holds for any machine.

Yeah the question was asked in the context of AAS, but it's just general scientific practice. Was pretty vague

[blacksanta62]

Hey guys, hoping someone can explain why the mono-hydrate shouldn't worry me as much as it is rn. And I have looked at the examiner reports but I still can't make sense of it