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September 19, 2020, 10:08:11 pm

Author Topic: QCE Chemistry Questions Thread  (Read 4284 times)

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Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #30 on: April 11, 2020, 11:28:04 am »
+2
Hi!
I have a few questions about my Chemistry Student experiment I am doing at the moment that relates to how I should conclude in regard to the validity of the experimental relationship and what relationship my experimental data is demonstrating; taking in consideration the percentage error in the gradient and possible faults in equipment.

My experiment involved a Daniell cell where I changed the concentration of copper sulphate from 0.25M, 0.5M, 0.75M and 1M and measured the voltage produced. The temperature was at 24 degrees and I used the Nernst equation to obtain the theoretical voltages. The Zinc sulphate was kept constant at 0.1M.

I just thought it would be easier to write my questions on a word doc since they are a bit long and so that I can put my graphs next to my questions.
Thank you so much!  :D

Hi!

I think you should revise your understand of some of the terminology to make sure you're using that accurately but there's definitely some great scientific thinking in there :D

Hope you find this helpful!

A.Rose

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Re: QCE Chemistry Questions Thread
« Reply #31 on: April 16, 2020, 03:09:26 pm »
+1
Hi!
I really really need some help with finding the concentration uncertainty for my dilutions!! The image I attached has my calculations and info regarding equipment uncertainty.
I am not confident that I am doing it right.
Basically, I began with 1M of copper sulphate so this is C1. And I want to make the following concentrations: 0.25M, 0.5M, 0.75M and 1M so all of these are C2. Then I want all of these solutions to have the same volume of 0.25L so this is V2. And hence the V1 volumes are 0.0625L, 0.125L, 0.1875L and 0.25L respectively for each concentration stated above.

I used the following equipment with its uncertainty; volumetric pipette (0.00075L), cylinder (0.0005L) and a volumetric flask (0.0003L).
V1 was measured I believe using the pipette and a cylinder. This was then all put into a 250mL volumetric flask and distilled water was added to bring the total volume (V2) to 0.25L.
I am not sure which uncertainties each V1 and V2 actually have when I go about plugging all the values into C1V1=C2V2 to achieve each C2 value with its correct concentration uncertainty.
Particularly - do I include the uncertainty of the flask if the uncertainty of V2 is based purely on the uncertainty of V1 and the distilled water added? (In my new calculations attached I haven't included the flask).

Thank you so much!
This is a small part of my student experiment that is due in a couple of days so help would be very much appreciated!!  :D :D
« Last Edit: April 17, 2020, 10:27:31 pm by A.Rose »

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #32 on: April 17, 2020, 10:35:06 pm »
+4
Hi!
I really really need some help with finding the concentration uncertainty for my dilutions!! The image I attached has my calculations and info regarding equipment uncertainty.
I am not confident that I am doing it right.
Basically, I began with 1M of copper sulphate so this is C1. And I want to make the following concentrations: 0.25M, 0.5M, 0.75M and 1M so all of these are C2. Then I want all of these solutions to have the same volume of 0.25L so this is V2. And hence the V1 volumes are 0.0625L, 0.125L, 0.1875L and 0.25L respectively for each concentration stated above.

I used the following equipment with its uncertainty; volumetric pipette (0.00075L), cylinder (0.0005L) and a volumetric flask (0.0003L).
V1 was measured I believe using the pipette and a cylinder. This was then all put into a 250mL volumetric flask and distilled water was added to bring the total volume (V2) to 0.25L.
I am not sure which uncertainties each V1 and V2 actually have when I go about plugging all the values into C1V1=C2V2 to achieve each C2 value with its correct concentration uncertainty.
Particularly - do I include the uncertainty of the flask if the uncertainty of V2 is based purely on the uncertainty of V1 and the distilled water added? (In my new calculations attached I haven't included the flask).

Thank you so much!
This is a small part of my student experiment that is due in a couple of days so help would be very much appreciated!!  :D :D

I checked your v1 calculations and they're all good but I'm not sure why when calculating the uncertainty you have V2 as .3 when you have it as .25 elsewhere and this is what the concentration calculations are based on. Additionally, you don't seem to have an uncertainty value for your initial concentration which won't be 100% precise so I would recommend also considering the uncertainty from this. I'm guessing that the pipette was used to transfer solution to the cylinder as it approached the meniscus rather than to measure it, in which case you wouldn't need to factor in the pipette's uncertainty. Other than my concerns about your initial uncertainties it looks like you've propagated uncertainty well :) 

Ok I typed all that before the updated edit so I'm typing again now:


For V2 uncertainty - if the water was added based on the location of the line and meniscus rather than measuring out a specific quantity of water and adding that - the uncertainty should be based on the equipment rather than being derived.

Edit: looks like you fixed the v1 issue, I've made slight wording changes for clarity
« Last Edit: April 17, 2020, 10:39:46 pm by Bri MT »

aleenabinu

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Re: QCE Chemistry Questions Thread
« Reply #33 on: April 19, 2020, 10:12:49 pm »
0
Hi,
For my SEI, i am doing determination of Vitamin C concentration in Fresh Lemon and Lemon juice using Iodine solution by titration.
I did the calculations and got very small numbers 0.0302M (for fresh lemon) and 0.0181M (for lemon juice). I have a feeling i got it wrong in finding the moles of ascorbic acid section because i'm unsure of the balanced equation.
I've watched few videos on how to do the calculations but they all used Potassium iodate and thats like a different equation.
This is the equation i used and hence got 1:1 mole ratio:
C6H8O6 + I2 --> 2I- + C6H6O6 + 2H+

sweetcheeks

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Re: QCE Chemistry Questions Thread
« Reply #34 on: April 19, 2020, 10:35:36 pm »
+5
Hi,
For my SEI, i am doing determination of Vitamin C concentration in Fresh Lemon and Lemon juice using Iodine solution by titration.
I did the calculations and got very small numbers 0.0302M (for fresh lemon) and 0.0181M (for lemon juice). I have a feeling i got it wrong in finding the moles of ascorbic acid section because i'm unsure of the balanced equation.
I've watched few videos on how to do the calculations but they all used Potassium iodate and thats like a different equation.
This is the equation i used and hence got 1:1 mole ratio:
C6H8O6 + I2 --> 2I- + C6H6O6 + 2H+

I wouldn't be so sure that those results are too low. What are you comparing them to?

Think about the pH of lemons and lemon juice and relate that to the H+ concentration. Keep in mind, ascorbic acid is a weak acid, and the pH is buffered, but none-the-less you can get an approximate value.

aleenabinu

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Re: QCE Chemistry Questions Thread
« Reply #35 on: April 21, 2020, 05:14:32 pm »
0
I wouldn't be so sure that those results are too low. What are you comparing them to?

Think about the pH of lemons and lemon juice and relate that to the H+ concentration. Keep in mind, ascorbic acid is a weak acid, and the pH is buffered, but none-the-less you can get an approximate value.

Thank you for replying!
I am comparing them to the recommended daily amount.
These are my calculations. Could you please tell me if i went wrong in any places? Thank youuu :)

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #36 on: April 22, 2020, 01:22:58 pm »
+5
Thank you for replying!
I am comparing them to the recommended daily amount.
These are my calculations. Could you please tell me if i went wrong in any places? Thank youuu :)

Hey!

There's some places where I can't check your work because, for example, I don't know what your concordant titres actually were.

One thing that did stand out is the concentrations of iodine you plugged into n=cv. Was this 0.005 M or 5 M? The reason you divide the volume by 1000 for the titres is because they're in mL and you need to convert them to L; if your concentration was 5.00 M you should not be dividing that by 1000.  Aside from that, I can't see any alarm bells in your calculations, though my personal preference would be for your molar ratio (the unknown/known fraction) to be specific to the molecules you were working with.

Good luck & I hope you found this helpful :)

aleenabinu

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Re: QCE Chemistry Questions Thread
« Reply #37 on: April 25, 2020, 05:54:44 pm »
0
Hey!

There's some places where I can't check your work because, for example, I don't know what your concordant titres actually were.

One thing that did stand out is the concentrations of iodine you plugged into n=cv. Was this 0.005 M or 5 M? The reason you divide the volume by 1000 for the titres is because they're in mL and you need to convert them to L; if your concentration was 5.00 M you should not be dividing that by 1000.  Aside from that, I can't see any alarm bells in your calculations, though my personal preference would be for your molar ratio (the unknown/known fraction) to be specific to the molecules you were working with.

Good luck & I hope you found this helpful :)

Thank you!! My teacher checked it and she said it looks fine.
One more question. Im comparing my concentrations to the recommended daily amount. I just realised that they're all in mg. I know you can convert concentration to mg, but that would just be for that quantity of sample. Is there a way i could convert it and obtain more accurate results?

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #38 on: April 25, 2020, 07:33:17 pm »
+2
Thank you!! My teacher checked it and she said it looks fine.
One more question. Im comparing my concentrations to the recommended daily amount. I just realised that they're all in mg. I know you can convert concentration to mg, but that would just be for that quantity of sample. Is there a way i could convert it and obtain more accurate results?


No worries!

hmmm... I'm not sure what question you're trying to answer, but you could consider converting the RDI to mol, then use that to find the volume of lemon juice required to meet the RDI based on your concentration calculation. This wouldn't make your results more accurate but it could increase your ability to analyse the relationship in a valid way

aleenabinu

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Re: QCE Chemistry Questions Thread
« Reply #39 on: April 28, 2020, 11:16:02 pm »
+1
No worries!

hmmm... I'm not sure what question you're trying to answer, but you could consider converting the RDI to mol, then use that to find the volume of lemon juice required to meet the RDI based on your concentration calculation. This wouldn't make your results more accurate but it could increase your ability to analyse the relationship in a valid way

thank you sm!! this helped alot.
as i am just determining the concentration of ascorbic acid in fresh lemon and bottled lemon juice, would i need to graph anything? now that i think about it, it wouldve been smart to record the pH but we didn't. or could i just insert a table with intial, final readings and tire volume and still get top marks in that criteria?

is iodine a weak or strong base? i can't find an exact answer anywhere :(

also, when explaining about the relationship, what actually is the relationship in this case? as more standard solution is added, more closer to the end point ? i dont knowww :-\
« Last Edit: April 28, 2020, 11:21:27 pm by aleenabinu »

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #40 on: April 29, 2020, 04:02:15 pm »
+1
thank you sm!! this helped alot.
as i am just determining the concentration of ascorbic acid in fresh lemon and bottled lemon juice, would i need to graph anything? now that i think about it, it wouldve been smart to record the pH but we didn't. or could i just insert a table with intial, final readings and tire volume and still get top marks in that criteria?

is iodine a weak or strong base? i can't find an exact answer anywhere :(

also, when explaining about the relationship, what actually is the relationship in this case? as more standard solution is added, more closer to the end point ? i dont knowww :-\

Hey,

Based on the questions you've been asking I think you need to go back to the start of the experiment and ask yourself:
- what's my hypothesis?
- what are my dependent and independent variables?


When graphing a categorical independent variable against a continuous dependent variable I recommend a bar or column graph (technically could also go with a boxplot or strip plot but you tend not to see those in highschool)

I would suggest that volume of standard solution is not one of the variables you're interested in graphing :)

A quick search is telling me that iodine is a weak base

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Re: QCE Chemistry Questions Thread
« Reply #41 on: August 12, 2020, 07:46:53 am »
0
Hi, I was wondering if anyone could explain why aluminium is only soluble when soil is acidic. Thank you :)

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #42 on: August 12, 2020, 11:29:32 am »
+5
Hey!

I was able to find this paper: Relationships among pH, aluminum solubility and aluminum complexation with organic matter in acid forest soils of the Northeastern United States

In it they showed a few theoretical models including:

Al(OH)3(s) + H^+ <-> Al3+ + 3H2O

RAl^(3-x) + xH^+ <-> Al^3+ + RHx    (where R^x- is an organic soil site)

and a version where x was specified as 3

RAl + 3H^+ <-> Al^3+ + RH3

In each of these you can see that the H+ is on the opposite side of a reversible reaction with the aluminium ions. As pH decreases, by Le Chatlier's  principle the system responds by favouring the forwards reaction thereby producing more Al3+


I hope this helps!

keltingmeith

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Re: QCE Chemistry Questions Thread
« Reply #43 on: August 12, 2020, 11:46:57 am »
+3
Legit, in the time it took me to come to this topic, Bri beat me with an answer :') But there is something I want to add:

While Bri's answer is fantastic, you might notice that it relies on scientific literature from 2016 - that's REALLY REALLY recent. Here's a list of some of the other things you have learned/will learn this year (assuming you're in year 12?) and when they were discovered:

Equilibrium - 1803
Galvanic Cells - 1790
Bronsted-Lowry Acid-Base Model - 1923
The pH Scale - 1909
Le Chatelier's Principle - 1884
NMR - 1945 (was awarded the Nobel prize in physics in 1952, neat)
Markovnikov's Rule - 1869
...

I think you get the point. You're studying old shit (the most recent of these is over half a century old!!) - and it's a little unreasonable for you to be read up on current-day research. But aside from that, there's something to note - a lot of what you're studying in your curriculum and textbooks ISN'T informed by current-day research. Solubility is a big one of those.

The truth with solubility is that in general - we DON'T know how it works. We can make guesses, such as why are silver halides insoluble if halides are typically soluble? Well, it would be because silver cations are insoluble. But wait - then why are silver cations insoluble? Well, the answer is going to be in one of:

-Silver-water interaction energy
-Silver-halide interaction energy
-The entropy of a silver-dissolved system (everything is soluble if you get it hot enough, even if that means taking it to infinity degrees)

The question is - which one? And the answer is typically - we don't know, but we can use one of these things to figure it out:

-Equilibrium constants
-Electrochemical potentials
-Actual chemical reactions that might take place (eg, metal oxides that are typically insoluble will be dissolved if you add enough acid, because the acid actually causes the oxides to chemically turn into water)

But all of these are typically just consequences of one of the above three things - interaction energy (or enthalpy) of one of the complexes, or the entropy of the system.

TL;DR - the answer to most solubility questions is basically, "we don't know, the answer is complicated", and hence why we need modern-day research like the article that Bri found to inform these kinds of questions. So, you shouldn't expect to be asked these questions at high school level (unless your textbook can offer you an answer) - but if you're interested for the sake of pure interest, then go nuts - I'll be the last person to discourage reading scientific literature for the fun of it.

EDIT: See below for Bri showing me up with my QCE knowledge . So yeah, turns out NMR isn't in the curriculum, oops. And I was unaware that QCE has a research investigation - looks damn cool though, I gotta say!
« Last Edit: August 12, 2020, 12:34:40 pm by keltingmeith »
Currently Undertaking: Doctor of Philosophy (PhD) in Supramolecular Photochemistry (things that don't bond but they do and glow pretty colours)

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Try out my study score calculator, request your subjects, and help give feedback if you've already completed VCE!

Bri MT

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Re: QCE Chemistry Questions Thread
« Reply #44 on: August 12, 2020, 12:06:33 pm »
+3
Legit, in the time it took me to come to this topic, Bri beat me with an answer :') But there is something I want to add:

While Bri's answer is fantastic, you might notice that it relies on scientific literature from 2016 - that's REALLY REALLY recent. Here's a list of some of the other things you have learned/will learn this year (assuming you're in year 12?) and when they were discovered:

Equilibrium - 1803
Galvanic Cells - 1790
Bronsted-Lowry Acid-Base Model - 1923
The pH Scale - 1909
Le Chatelier's Principle - 1884
NMR - 1945 (was awarded the Nobel prize in physics in 1952, neat)
Markovnikov's Rule - 1869
...


Don't remind me about how the QCE system doesn't include NMR in the syllabus :'(

Quote from:  QCAA
Analytical techniques
explain how proteins can be analysed by chromatography and electrophoresis
select and use data from analytical techniques, including mass spectrometry, x-ray crystallography and infrared spectroscopy, to determine the structure of organic molecules 
analyse data from spectra, including mass spectrometry and infrared spectroscopy, to communicate conceptual understanding, solve problems and make predictions. 

Aside from that reminder, always good to see your additions :)

TL;DR - the answer to most solubility questions is basically, "we don't know, the answer is complicated", and hence why we need modern-day research like the article that Bri found to inform these kinds of questions. So, you shouldn't expect to be asked these questions at high school level (unless your textbook can offer you an answer) - but if you're interested for the sake of pure interest, then go nuts - I'll be the last person to discourage reading scientific literature for the fun of it.

I'm assuming Jasmine that you're looking at this for SHE and/or IA3? Absolutely if you ever got something like that question on an exam or test without them providing the required information that would be very unreasonable.