Ok, for 126 Lab 9 next week - you MUST calculate your buffer for pH 4.5, and you WILL be required to re-do that calculation for the pH you are assigned. That means GET IT RIGHT and COME TO OFFICE HOUR. We will be happy to tell you if something is wrong.
Know the Henderson-Hasselbalch equation, and in the last question, the total [H+] muist be worked out to correctly determine whether the acid or base is in excess.
65% is the highest midterm average of any of my midterms over the last two semesters (Chem 121 were both around 50%, Math is usually around 50%, Bio was 56% this semester)
If you're used to arts or business where the average is 80-90%, you'll be in for a culture shock in the sciences. In general midterm and final exam averages run anywhere from 30% to 70%, and as a result scaling benefits the "average" person far more than in business or the arts.
You're solving for that ratio in the log. Put 4.5 on the left, 4.74 for the pKa, isolate the log, you get -0.24, take inverse logs of both sides. At that point you should get a ratio of 0.57 to 1. Define the volume of the CH3COOH component as x and therefore anything not CH3COOH is CH3COO-, so that's 60-x
You get 0.57 = 60 - x / x. Solve.
Remember: The acid and base components of a buffer titrate essentially independently of each other, so you need not worry about the presence of the other component when you work out titration information.
You're solving for that ratio in the log. Put 4.5 on the left, 4.74 for the pKa, isolate the log, you get -0.24, take inverse logs of both sides. At that point you should get a ratio of 0.57 to 1. Define the volume of the CH3COOH component as x and therefore anything not CH3COOH is CH3COO-, so that's 60-x
You get 0.57 = 60 - x / x. Solve.
Remember: The acid and base components of a buffer titrate essentially independently of each other, so you need not worry about the presence of the other component when you work out titration information.
Hey Nukechem,
Does Henderson-Hasselbach equation only apply to buffers or to all acids and bases?
KNOW the ion-electron (aka half-reaction) method of balancing redox reactions. Balancing by oxidation numbers will get you the hairy eyeball.
Appendixes 1, 3, 4, 9, and 10 will be very helpful. In particular PLEASE NOTE that we have taken pity on you and said you do not have to balance the thiocyanate reactions because the products are very ugly.
Be aware of some of the subtleties of the tests you are doing. A key feature will often be a color change, precipitation, or how fast or slow a reaction occurs. So don't read your lab manual the hour before the lab. Read it the day before and let it sink in to your head so that you have a bit of an intuitive feel for how you will be proceeding.
Specific tests are often a judgement call. I suggest doing all of the easy ones, and then deciding if you need to do the harder ones. :tongue:
Yeah, there's a final exam. It's in the Gym (not sure exactly where that is, but I will find out and get the directions, unless someone reading this knows where already and can give them), on April 12 at 3:30 PM. <-- SUBJECT TO CHANGE IF I GET THE DATE WRONG.
For reference, the 122 final is on Saturday April 14 at 3:30 PM in either of B9200 or B9201 depending on last name, IIRC.
It works like Experiment 3 or 5; you will need to have a spreasheet set up and ready to go, as this week you WILL be submitting electronically. So make sure it WORKS and come to office hour if you need help getting it to work.
To this end, DO THE PRELAB. It will take you through the same sort of steps that you will do in the lab yourself. Additionally, please be aware that you will not construct a calibration graph. You'll measure a standard ONCE and then use it in the ratio of absorbances to get a concentration. What you are effectively doing is what we call a "single-point" calibration - that is, we assume that the single data point you use in the standard lies exactly along the calibration curve.
First of all, I found a rather handy study guide with do-it-yourself problems and an answer key at the back so you can check yourself. It's a PDF file, linked below:
Second, another PDF to read - check the last page of lecture 19 for CHEM 122. It has a handy little table to explain the qualitative behavior of the Gibbs free energy equation.
Essentially the Gibbs Free Energy expresses the competition or cooperation (subject to partial temperature dependence) of the enthalpy and entropy of a chemical reaction (which is why if it equals zero the rxn is in equilibrium, as neither factor can cause it to go to completion. In addition the effects of a positive/negative entropy change and a positive/negative enthalpy change will be different since they deal with different thermodynamic properties (lowering the energy of a system versus achieving a higher state of disorder).
1. Understand redox reactions and the sign conventions for how to add them depending on whether you write the half-reactions as reductions or whether you write one as an oxidation and one as a reduction.
2. The number 96485 Coulombs per mole of electrons may seem like an all-purpose number, but in actual fact two numbers are multiplied together to get this. Once you realize this you will understand why I will now tell you in bold letters not to use it in the last part of the lab when you are calculating an experimental Avogadro's number. It is fine to use it in the prelab.
3. You will actually have to know some physics! Q (charge transferred) = I (current flow) * t (time) - you may also find it helpful to know that Q = N (number of electrons) * e (the fundamental unit of charge).
4. As always if you have trouble or don't understand something, come to office hour. That's what we TAs get paid for.
I'm not Bryan, but he should have been in C8039 at 10 AM. You didn't try going to C8033 or the lab room by mistake, did you?
I think Leon or one of the other TAs shows up at 11. I show up on Mondays sometimes, but in an unofficial capacity since it's not my usual office hour (and rarely, at that).
I'm not Bryan, but he should have been in C8039 at 10 AM. You didn't try going to C8033 or the lab room by mistake, did you?
I think Leon or one of the other TAs shows up at 11. I show up on Mondays sometimes, but in an unofficial capacity since it's not my usual office hour (and rarely, at that).
My official office hour is Friday at 10 AM.
merci beaucoup.too bad i learned about this on the last two labs. the office hours.
In particular for electrochemistry stuff - PLEASE READ lecture 21 of the CHEM 122 notes on the SFU Chemistry website. Note that the sign convention adopted by Dr. Walsby requires writing both half-reactions as REDUCTIONS.
If you write one half-reaction as a reduction and the other as an oxidation, the E-zero values ADD, not subtract. This is why I was being anal about sign conventions in my previous hints. :smile:
A word on 126 with Dr. Canal - I've TAed for him before. He's cool.
:smile:
So don't worry that he'll be horrible or something - he likes to be short and to the point when it comes to lab lecture, so you should get to lab on time if you want to pick up any pointers before you start lab.
Comments
Know the Henderson-Hasselbalch equation, and in the last question, the total [H+] muist be worked out to correctly determine whether the acid or base is in excess.
Are the midterms marks usually curved?
I think the class avg on this one is 65%
Can you post some office hour times for tomorrow?
http://www.sfu.ca/chemistry/students/courses/chem126/index.htm
:smile:
Re: Science marks.
If you're used to arts or business where the average is 80-90%, you'll be in for a culture shock in the sciences. In general midterm and final exam averages run anywhere from 30% to 70%, and as a result scaling benefits the "average" person far more than in business or the arts.
You're solving for that ratio in the log. Put 4.5 on the left, 4.74 for the pKa, isolate the log, you get -0.24, take inverse logs of both sides. At that point you should get a ratio of 0.57 to 1. Define the volume of the CH3COOH component as x and therefore anything not CH3COOH is CH3COO-, so that's 60-x
You get 0.57 = 60 - x / x. Solve.
Remember: The acid and base components of a buffer titrate essentially independently of each other, so you need not worry about the presence of the other component when you work out titration information.
Does Henderson-Hasselbach equation only apply to buffers or to all acids and bases?
thanks
KNOW the ion-electron (aka half-reaction) method of balancing redox reactions. Balancing by oxidation numbers will get you the hairy eyeball.
Appendixes 1, 3, 4, 9, and 10 will be very helpful. In particular PLEASE NOTE that we have taken pity on you and said you do not have to balance the thiocyanate reactions because the products are very ugly.
Be aware of some of the subtleties of the tests you are doing. A key feature will often be a color change, precipitation, or how fast or slow a reaction occurs. So don't read your lab manual the hour before the lab. Read it the day before and let it sink in to your head so that you have a bit of an intuitive feel for how you will be proceeding.
Specific tests are often a judgement call. I suggest doing all of the easy ones, and then deciding if you need to do the harder ones. :tongue:
If the attachment-thing works you should be able to see the jpeg and the pdf. Use whichever is convenient. :)
the electrons get a bit tricky. thats it.
and is there an exam for the lab compenent 126?
For reference, the 122 final is on Saturday April 14 at 3:30 PM in either of B9200 or B9201 depending on last name, IIRC.
It works like Experiment 3 or 5; you will need to have a spreasheet set up and ready to go, as this week you WILL be submitting electronically. So make sure it WORKS and come to office hour if you need help getting it to work.
To this end, DO THE PRELAB. It will take you through the same sort of steps that you will do in the lab yourself. Additionally, please be aware that you will not construct a calibration graph. You'll measure a standard ONCE and then use it in the ratio of absorbances to get a concentration. What you are effectively doing is what we call a "single-point" calibration - that is, we assume that the single data point you use in the standard lies exactly along the calibration curve.
Some more notes...
First of all, I found a rather handy study guide with do-it-yourself problems and an answer key at the back so you can check yourself. It's a PDF file, linked below:
http://www.hwscience.com/Chemistry/regchemistry/Unit%209/Unit%209%20Study%20Guide.pdf
Second, another PDF to read - check the last page of lecture 19 for CHEM 122. It has a handy little table to explain the qualitative behavior of the Gibbs free energy equation.
Essentially the Gibbs Free Energy expresses the competition or cooperation (subject to partial temperature dependence) of the enthalpy and entropy of a chemical reaction (which is why if it equals zero the rxn is in equilibrium, as neither factor can cause it to go to completion. In addition the effects of a positive/negative entropy change and a positive/negative enthalpy change will be different since they deal with different thermodynamic properties (lowering the energy of a system versus achieving a higher state of disorder).
Ok, some notes -
1. Understand redox reactions and the sign conventions for how to add them depending on whether you write the half-reactions as reductions or whether you write one as an oxidation and one as a reduction.
2. The number 96485 Coulombs per mole of electrons may seem like an all-purpose number, but in actual fact two numbers are multiplied together to get this. Once you realize this you will understand why I will now tell you in bold letters not to use it in the last part of the lab when you are calculating an experimental Avogadro's number. It is fine to use it in the prelab.
3. You will actually have to know some physics! Q (charge transferred) = I (current flow) * t (time) - you may also find it helpful to know that Q = N (number of electrons) * e (the fundamental unit of charge).
4. As always if you have trouble or don't understand something, come to office hour. That's what we TAs get paid for.
http://www.sfu.ca/chemistry/students/courses/chem126/exp_pages/exp12.html
and if you're not bryan, when and where is the office hour for the TAs
I think Leon or one of the other TAs shows up at 11. I show up on Mondays sometimes, but in an unofficial capacity since it's not my usual office hour (and rarely, at that).
My official office hour is Friday at 10 AM.
Check the main 126 website for the new office hours.
http://www.sfu.ca/chemistry/students/courses/chem126/index.htm
In particular for electrochemistry stuff - PLEASE READ lecture 21 of the CHEM 122 notes on the SFU Chemistry website. Note that the sign convention adopted by Dr. Walsby requires writing both half-reactions as REDUCTIONS.
If you write one half-reaction as a reduction and the other as an oxidation, the E-zero values ADD, not subtract. This is why I was being anal about sign conventions in my previous hints. :smile:
Moderators - can you keep this open for the summer students who'll be taking 126? They can use the hints.
:smile:
So don't worry that he'll be horrible or something - he likes to be short and to the point when it comes to lab lecture, so you should get to lab on time if you want to pick up any pointers before you start lab.