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July 02, 2020, 03:53:00 pm

Author Topic: QCE Chemistry Questions Thread  (Read 3268 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 »
+3
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 »
+4
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