Login | Register

Welcome, Guest. Please login or register.

September 27, 2020, 01:00:53 am

Author Topic: QCE Chemistry Questions Thread  (Read 4579 times)

0 Members and 1 Guest are viewing this topic.

Sine

  • National Moderator
  • ATAR Notes Legend
  • *****
  • Posts: 4493
  • Respect: +1552
Re: QCE Chemistry Questions Thread
« Reply #15 on: January 08, 2020, 05:41:55 pm »
+4
-snip-
I did a rough calculation before of this and got the same so I think it is right.

However, I should note that if this was a practical experiment whether this number is accurate would depend on how well you did your titrations.

Hope this helps :)

alphabeta

  • Adventurer
  • *
  • Posts: 21
  • Class of 2020
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #16 on: January 08, 2020, 07:32:53 pm »
0
Quote
I did a rough calculation before of this and got the same so I think it is right.

However, I should note that if this was a practical experiment whether this number is accurate would depend on how well you did your titrations.

Hope this helps :)

Thanks Sine! Really appreciate your time. :)

alphabeta

  • Adventurer
  • *
  • Posts: 21
  • Class of 2020
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #17 on: February 02, 2020, 12:16:05 pm »
0
Hi all - me again!

Just a quick question:

Is 'A being titrated with B' the same as 'A being titrated against B? In both cases, is the substance of unknown concentration the same?

Thanks!  :D

Erutepa

  • VIC MVP - 2019
  • Victorian Moderator
  • Forum Leader
  • *****
  • Posts: 676
  • evenin'
  • Respect: +702
Re: QCE Chemistry Questions Thread
« Reply #18 on: February 02, 2020, 05:12:51 pm »
+3
Hi all - me again!

Just a quick question:

Is 'A being titrated with B' the same as 'A being titrated against B? In both cases, is the substance of unknown concentration the same?

Thanks!  :D
I usually see it written as "A is titrated with B" (where B is the titrant/standard solution), however I am sure that saying A is titrated against B is the same and fine to say :)
Qualifications
 > Have counted to 227
 > Can draw really good spiders
 > 2 Poet points
 > 6.5 insanipi points
 > 1 Bri MT point

alphabeta

  • Adventurer
  • *
  • Posts: 21
  • Class of 2020
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #19 on: February 02, 2020, 07:38:38 pm »
+1
I usually see it written as "A is titrated with B" (where B is the titrant/standard solution), however I am sure that saying A is titrated against B is the same and fine to say :)
Thanks heaps Erutepa! I usually see it as A titrated with B as well.  :)

Also, I have stumbled across another problem. So, with titration graphs I know at the half-equivalence point the pH = pKa. However, I have also come across some more information that says the indicator changes colour when the pH = pKa, meaning that it wouldn't change at the end point. Can someone clear this up for me?

Thanks so much 

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 3971
  • invest in wellbeing so it can invest in you
  • Respect: +2924
Re: QCE Chemistry Questions Thread
« Reply #20 on: February 02, 2020, 10:40:58 pm »
+3
Hey,

The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.

At the half equivalence point, enough titrant (known concentration) has been added to the analyte (unknown concentration) that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.

These are different pKa s and thus it makes sense that they occur at different pH values.

If you're unclear about ether of these please feel free to ask :)

Edited for clarity
« Last Edit: February 04, 2020, 09:46:11 am by Bri MT »

alphabeta

  • Adventurer
  • *
  • Posts: 21
  • Class of 2020
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #21 on: February 03, 2020, 08:08:13 pm »
+1
Hey,

The indicator changes colour when pH = pKa. In this case, we're referring to the pKa of the indicator.

At the half equivalence point, enough analyte has been added that half of it is protonated and half of it is deprotonated, thus making pKa equal to pH.

These are different pKa s and thus it makes sense that they occur at different pH values.

If you're unclear about ether of these please feel free to ask :)

Thanks Bri MT!

So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?

I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?

Thanks so much again Bri, appreciate it. :)

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 3971
  • invest in wellbeing so it can invest in you
  • Respect: +2924
Re: QCE Chemistry Questions Thread
« Reply #22 on: February 04, 2020, 10:37:09 am »
+4
Thanks Bri MT!

So, just to make sure, in the first instance it is talking about the pKa of the indicator, and in the second instance, it is talking about the pKa of the unknown solution?

I really want to make sure I have a firm grip of the titration concepts, so are there any worksheets with simple and challenging titration problems I can do (preferably with worked solutions, if not I can confirm my answers with you)?

Thanks so much again Bri, appreciate it. :)

I've edited my above post for clarity since titration jargon can be confusing but yeah.

Let's say, for example, that you have NaOH in your burette (titrant, known concentration) and you're trying to figure out what the concentration of some acetic/ethanoic acid (analyte) is. To find the concentration of the acetic acid you want to identify the volume of NaOH required to reach the equivalence point, where the stoichiometric ratio is met. To visually approximate this, an indicator is added which  is a weak acid/base that will quickly change which conjugate is present in a greater quantity and, since each conjugate is a different colour, will change colour at that point. When the indicator transitions from one colour to another, there is the same amount of each conjugate present and the pKa (of the indicator) = pH (of the solution). In this example, your indicator might be something like phenolphthalein

Before the equivalence point is reached, there will be a stage where you've added sufficient titrant (in this example NaOH) that half of the analyte (in this case acetic acid) is protonated and half is deprotonated - in other words, half is the conjugate acid and half is the conjugate base. Thus, we are looking at another situation where pKa =pH but this time it's the pKa of our acetic acid since that's what has a 50/50 balance with conjugate base & conjugate acid. For half of our acetic acid to be deprotonated, the volume of NaOH required is half that needed to fully deprotonate our acetic acid in accordance with the equation NaOH + CH3COOH -> H2O + CH3OONa . I.e. the v(NaOH) is half of what v(NaOH) is at the equivalence point.

Remembering that the stoichiometric ratio is met at the equivalence point (which we approximate with the end point, where the indicator changes colour), this is why pH = pKa at the half equivalence point for the acetic acid and is at the equivalence point for the indicator.

Hope this clarifies things!


I don't think I have a worksheet like that on hand but I'll look around and see if I can find any good question sets like that for you. If you haven't already, you might find it useful to do the maths for why pH = pKa when each conjugate is present in the same amount to solidify your understanding.

A.Rose

  • Trailblazer
  • *
  • Posts: 32
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #23 on: February 18, 2020, 04:35:31 pm »
+1
Hello
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.

In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?

If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 3971
  • invest in wellbeing so it can invest in you
  • Respect: +2924
Re: QCE Chemistry Questions Thread
« Reply #24 on: February 20, 2020, 10:47:07 am »
+3
Hello
I am a year 12 Chemistry student and I was wondering if you could help me with some questions I have about a Galvanic cell experiment I am doing for my student experiment.
The experiment I decided to do was to change the concentration of the copper sulphate electrolyte in a Daniell cell and measure the voltage produced. The other electrolyte; Zinc nitrate was not changed and kept constant at 0.1M. The copper sulphate concentration increased from 0.25M, 0.5M, 0.75M to 1M. The experiment was performed when the temperature was 24 degrees. The anode was Zinc and the cathode was Copper.

In order to obtain theoretical values - should I use the Nernst equation? Would you be able to tell me how to use the Nernst equation for my experiment?
Also, I understand that voltage increases with concentration, but compared to standard conditions should the voltage produced be higher or lower than the voltage produced at standard conditions and 1M for each electrolyte?

If you can help me that would be absolutely amazing and this is a really important assignment.
Thanks!!

Hey :)

Welcome to the forums!

The Nernst equation is applicable here since you're working with non-standard conditions :).  I'm guessing what might be confusing you in the calculation is Q? Q is calculated using the same formula as Kc but we use Q (reaction quotient) rather than Kc (equilibrium constant) if the system isn't at equilibrium. Please also make sure you remember to convert your temperature to Kelvin before plugging it in.

When we apply the Nernst equation to your set up, Q is what is changing between experiments so let's look at that. For Q = [Zn2+]/[Cu2+], we see that Q increases as [Cu2+] decreases. This suggests that your voltage will decrease as [Cu2+] decreases.


Hope this helps, please feel free to ask any follow up questions & best of luck with your assignment :)

A.Rose

  • Trailblazer
  • *
  • Posts: 32
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #25 on: March 20, 2020, 07:52:42 pm »
0
Hello!

I have just started Organic chemistry and I have a quick question on how to draw 4-oxo propyl hexanoate. (The answer from my textbook is attached) I understand the carboxyl group and everything just not the placement of the ketone. Isn't carbon 1 the carbon with the carboxyl group attached? In the answer, I don't know why the ketone is where it is if carbon 1 is where the carboxyl group is. The answer seems to have carbon 1 as the very end carbon...?

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 3971
  • invest in wellbeing so it can invest in you
  • Respect: +2924
Re: QCE Chemistry Questions Thread
« Reply #26 on: March 23, 2020, 11:55:02 am »
+3
Hello!

I have just started Organic chemistry and I have a quick question on how to draw 4-oxo propyl hexanoate. (The answer from my textbook is attached) I understand the carboxyl group and everything just not the placement of the ketone. Isn't carbon 1 the carbon with the carboxyl group attached? In the answer, I don't know why the ketone is where it is if carbon 1 is where the carboxyl group is. The answer seems to have carbon 1 as the very end carbon...?

Hey,

Counting starts from the end of a molecule and with the choice of what end being made to lower the numbers you use. I think where you may be getting confused here is reading this as [4-oxo-propyl][hexanoate] and thus counting from the hydroxyl end of the propanol, with that end of the molecule going to the ester functional group. Note in in this question you can't interpret it that way due to the limited number of carbons in propanol.

In this question you need to read it as [4-oxo] [propyl hexanoate]. I.e. the ketone functional group needs to be 4 carbons from the end. You can only do this in the way shown in the textbook answer.


Hope this makes sense!

A.Rose

  • Trailblazer
  • *
  • Posts: 32
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #27 on: April 03, 2020, 10:16:14 am »
0
Hello
Would someone be able to check my calculation using the Nernst equation? See attached:
Thank you so much!!   :D

Bri MT

  • VIC MVP - 2018
  • Administrator
  • ATAR Notes Legend
  • *****
  • Posts: 3971
  • invest in wellbeing so it can invest in you
  • Respect: +2924
Re: QCE Chemistry Questions Thread
« Reply #28 on: April 07, 2020, 11:51:58 am »
+2
Hey,

Plugging in the same values as you used my calculator gave me a slightly different value (1.12016 V) so maybe double check your calculation checking for:
- reading the decimal places carefully
- having a x symbol between the fraction and the log / not using the simplified fraction form. (Some calculators read 3 1/2 as 7/2 or 3 + 1/2  rather than as 3/2 )


Note that some of the values you used had more accuracy than is published in the QCAA formula and data book and so using them for practice questions may produce slightly different results than you would see in suggested solutions

Finally, I recommend losing some of your decimal places as they imply much more accuracy in the data than what you actually have.


Hope this helps!

A.Rose

  • Trailblazer
  • *
  • Posts: 32
  • Respect: 0
Re: QCE Chemistry Questions Thread
« Reply #29 on: April 09, 2020, 11:58:59 am »
0
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