2046lab advice

8/13/2019 2046Lab Advice

http://slidepdf.com/reader/full/2046lab-advice 1/6

2046Lab

I had been taking chem courses at UF for a year and had taken almost 2 years of chemistry at one of the

most prestigious secondary schools in the world before I took CHM2046L at UF. Nonetheless I still had

no idea what was going on in that class a good portion of the time. Its not so much what it is you are

learning you as much as how it is Horvath's book is going about teaching it to you, the lab manual is hardto read. You only have so much time to finish that book and along the way you have all the unknowns

you have to do within your lab sessions. So here are some tips that helped me out:

-Find your unknown schemes on the internet. For real, they are everywhere and have been passed down

again and again. There is only 1 way to write the schemes 100% correctly for each unknown and it is not

worth the waste of time and strain to try and do it yourself from the book and have it constantly

corrected and looked at by your TA or CLC lab people so that it is right. Just change the

order/font/words around enough to make it look original.

-Race through the book. When in lab you are in high gear, I dreaded this class because it was 3 hoursnonstop. When Dave tells you to clean up... keep going until the last minute. They (the lab managers)

only care about getting you out and the next people in so they can go home. They do NOT care about

your grade. Only your TA will, and that is if he/she is nice. You want/need to be able to finish at least

some extra assignments at the end so that you can get extra credit to not have to take the final and

make up for a bad unknown.

-Rehearse unknowns at home before you do them. Mentally think about and jot down notes about what

you are going to do before you do it. Unknowns are crunch time. You have to do everything right and

everything right the first time. Keep as much stuff on your counter as possible so that you are not always

going to the rack.

-Don't make dilutions if you can get away with it. Seriously, i didn't dilute 70% of the time outside of

unknowns. I would just uncork that bad boy 6M NaOH and use it straight. The only reason the book says

to is to save chemicals. Most of the time you are looking for the presence or absence of a precipitate

and whether or not the solution you are adding is 1M or 6M won't make a difference, in fact the 6M will

probably make a more noticeable precipitate. Though you should not do this when you are working with

amphoteric precipitates that precipitate and also are dissolved with the same chemical. You will be able

to pick up on the times when you can skip diluting.

-In general read ahead and read the first parts of each chapter. This will just expand your knowledge

about what is going on and why you are doing what the book tells you to do. This will also help you to

spot the multiple times the book tells you to get small samples of A, B, C, and D and treat all of them

with X and on the next page it tells you to AGAIN get samples of A, B, C, and D and treat them with Y

(the book is designed to waste your time because some people in lab have to not finish so not everyone

gets a good grade). Well after you treat the first set of samples with X you can't treat them with Y, they

have been used. Couldn't you tell me BEFORE to get two samples each of ABCD to save my time so I

8/13/2019 2046Lab Advice


wouldn't have to go back the rack and waste more time? You can't just read as you go along if you don't

want to fall victim to these nuisances.

-As far as the actual lab work, assignments, and homework goes; do your best, try to give as best

answers you can give. It is so much to read that most TAs just make sure you give good answers so you

can get credit. I remember when I wasnt sure what to write I would use "Precipitate A forms and not B

because A is more stable"; I didn't know the chemistry behind it but I would just put a logical answer

that sounds good. Use the stuff posted outside CLB for guiding you on the HW problems.

The ease of this course will depend a lot on your TA, I was lucky to have one who had no problem

helping explain things and helped us a lot on our unknowns. I also had a really small section so a lot of us

helped each other out a lot and would split up the work on long assignments and work together even

though we were not allowed. I know this course sucks hard and the book is not well written because the

course coordinator is WAY out there with his love for chemistry. Its a lot of work for 1 credit.

Okay so knowing when you can and cannot dilute is a skill you will eventually hopefully master throughthe course, at least within the first 4 weeks. Just remember that a LOT of questions go as follows: “Add 3

drops of 1M of NaOH to 2 mLs of B. What forms? Write down, if any, the formula for the reaction and

describe its formation”. Well, 90% of the time something DOES form and 90% of that time the formation

of it is independent of the strength of NaOH. So what I would do is take a little vial, fill it about halfway

with B, then uncork the 6M NaOH and pour a little in the vial, or get my eyedropper and use it to put a

few drops in the vial. Precipitate will or will not form, there is no halfway, it’s yes or no. Soon you will

recognize situations like this and even in places where you might need to dilute you can do a quick half

and half 6M with water to make a 3M which is practically the same as 1M. But, in this example, just

think…. I saved time by not diluting, saved time by not using a graduated cylinder in getting exactly 2

mLs of B and I saved time not being confused by only adding 3 drops of 1M NaOH because that is SOOlittle that you may not even notice the precipitate. Like I said before, the book is designed to waste

YOUR time and to conserve THEIR chemicals. I didn’t feel one iota of guilt not diluting and using as much

chemical as I wanted from the shelves.

Probably, though, the most imperative thing, and I can’t stress it enough, is doing the coursework as fast

as you can. There are many ways to do this but way most students do of reading the book only while at

lab and reading/following the directions as they go along won’t cut it. Never leave lab early. Never skip

lab. If you are in the middle of an assignment when lab ends make sure you put your stuff away so you

can pick back up the next week as fast as you can. I was crunched for time but I know if I had read ahead

I would have saved even more time and the class wouldn’t have been such a stress fest. I would have

known that on the next page I would have to make a hot water bath so I could have made it earlier while

doing something else and not waste time waiting for water to boil. I would have known when I needed

to get more than one sample of the same chemical. I would have known that making, rinsing, washing,

re-dissolving, and collecting a “pure” sample of XXX that turned out to be Calcium Carbonate is a

complete waste of time when there is pure calcium carbonate in a bottle sitting on the shelf. Remember

that the EASIEST way for UF to curve a class is to make it so that some finish and some do not. This is the

8/13/2019 2046Lab Advice


exact same situation as PHY2053/PHY2054 labs.

Doing all the book and extra credit assignments WILL get you at LEAST a B+ but more than likely an A

because there is a ridiculous amount of extra credit in this course. I did ALL of the extra credit and had

an A so I didn’t have to take the final and on my 3 unknowns I got a 90, 70, and 65 percent respectively!

That is an average of a C grade on each one! And I could have gotten even worse on my unknowns and

had to take the final but only would need 30-40 percent on it!! I know some people who go into that

final (worth 120 points) only needing 5-10 points on it to get the grade they want.

And probably the other thing is about the unknown schemes. I know this is controversial, but the fact is

a large, large portion.. in fact probably the MAJORITY of students in this class are NOT writing those

things all by themselves. It is hard enough to read that book and figure out WHICH freaking ions you are

going to be tested on let alone how to quickly/effectively test for them. There are emails, facebook

pages, online resources, and past students you can get in touch with (me being one of them) available

for finding help with schemes and I personally suggest using such resources. I mean, you should not be

stupid enough to turn in someone else’s work exactly, but like I said before there is only exactly 1 way to

do the tests correctly so it’s not like every student who has ever created a scheme for 2046L has written

something revolutionary. Furthermore, having a correct scheme by no means guarantees you will do

your unknown correctly. You still have to read and follow the scheme without struggle. But at the same

time, if your scheme is *good* you do not have to think about the chemistry that is going on behind

each test and you can go through and say “present” or “not present” for each ion. I don’t feel guilty

using old schemes as I guide, I mean if the coordinators for this course have such a problem with it then

maybe they should not publish the same Lab Manual every semester for 4 years in a row. I know it is

logically fallacious to say “everyone does it”… but even the people who KNOW all about what is going on

I have seen fail their unknowns after writing their own scheme. I mean.. your unknowns ARE the

“exams” of this class, it is not worth the risk. This upcoming week would be Week 2 out of 13 and some

students will be doing their first unknown as early as Week 3 (easiest and most enjoyable unknown

btw.. it only gets harder from there) so if you plan on doing your unknown soon get your scheme and

know it well before you walk into lab.

Oh, and the homework answers are posted in a glass case on the north part of the east wall of CLB. If

you leave Library West facing south walk out towards the plaza a little bit the turn to the west and go

straight towards the building it is on that wall.

**SCHEME FOR UKNOWN 2:**

Unknown Scheme: 2

I. Description

A) Phase, solubility, color, odor, shape

II. Flame Test

8/13/2019 2046Lab Advice


A) Orange flame Na+ present (possibly K+, NH4+)

B) Red flame Ca2+ present (no Na+)

C) Purple flame K+ present (possibly NH4+)

D) No change in flame color Ca2+, K+, Na+ absent (possibly NH4 present)

III. pH Test

A) Add a little bit of unknown to a small amount of neutral water

B) Fill several vials with test solution and use indicators to determine pH

Common pHs for Anions and Cations:

pH 1.5: HSO4- present; CO32-, HCO3-, OH- not present

pH 2.5: SO42- present

pH 3-3.5: Al(H2O)63+ present; CO32-, HCO3-, OH- not present

pH 5-6: NH4+ and/or Mg(H2O)62+ present

pH 7: Cl- and/or NO3- and/or SO42- and/or K+ and/or Na+ and/or Ca2+ present

pH 9-10: HCO3- present; HSO4-, OH-, and Al(H2O)63+ not present

pH 10-12: CO32- present; HSO4-, Al(H2O)63+, NH4+ not present

pH 13: OH- present; NH4+, Al(H2O)63+, HSO4-, HCO3- not present

If unknown is an insoluble compound, use the liquid to test pH

pH 7: CaSO4*2H2O

pH 9.5: MgCO3

pH 10: CaCO3

pH 10.5: Mg(OH)2

pH 12.5: Ca(OH)2

*Separations are necessary to test for Cations. To Separate, add excess 3-6M NH3+ to separate Mg+ and

Al2+ from Na+, K+, NH4+, and Ca2+*

*Ca2+ Separation:

A) Treat sample solution with excess 0.5M Na2CO3

B) After ppt. has formed, centrifuge in test tube

C) Treat centrifuged sample with more Na2CO3 and repeat until Ca2+ is out

*Mg(H2O)2+/Al(H2O)3+:

A) If no ppt. forms after addition of NH3, then Mg(H2O)62+ is not present

B) If ppt. forms with NH3, but doesn’t form a ppt. with the addition of excess 2M NaOH, then

Mg(H2O)62+ is present and Al(H2O)3+ is absent

IV. NH4+ Test

A) Observe test results for pH test (if acidic, NH4+ could be present)

8/13/2019 2046Lab Advice


B) Place about 2-5mL of sample solution in vial and add an equal amount of 1M NaOH to form NH3

C) If smell of NH3 is present, then NH4+ is present and pH test will be that of a basic solution

*Removal of NH4+ (perform only if NH4+ test is positive)

A) Heat substance by placing a small portion of the solid unknown in an open crucible

B) Heat until no fumes or smoke are discharged

V. K+ Test

A) If NH4+ is present, be sure to remove it before testing for K+ by ppt.

B) In small vial, place remains of above test

C) Add 1 drop of 6M HAc and 2-3 drops of Cobaltnitrite reagent

D) If yellow ppt. forms, K+ is present

VI. Cl- Test

A) Place several drops of sample solution in vial and add 1M HNO3 until solution is acidic (test acidity

with red litmus paper)

B) Add one drop of 0.02M AgNO3

C) If cloudy ppt. forms, then Cl- is present

VII. SO42-/HSO4- Test

A) Place several drops of sample solution in vial and add 1M HNO3 until acidic (test acidity with red

litmus paper)

B) Add one drop of 0.25M Ba(NO3)2

C) If cloudy ppt. forms, then SO4- is present, unless CO32- is present

D) To distinguish between SO4- and HSO4-, test pH

VIII. NO3- Test

A) Place small crystal of Fe(OH2)6(NH4)2(SO4)2 in a spot plate

B) Cover crystal with a few drops of the sample solution

C) Immediately add 2 drops of 18M H2SO4

D) If brown solution forms, NO3- is present

IX. CO32-/HCO3- Test

A) Treat sample solution with 1M HCl to observe generation of colorless, odorless gas (CO2)

B) To distinguish between CO32- and HCO3-, test pH

C) If test is positive for CO2 and isn’t very soluble in H2O, it is CO32-

Possible Chemicals:

Cations: Ca2+, K+, Na+, NH4+, Mg (H2O)62+, Al (H2O)63+,

Anions: Cl-, NO3-, SO42-, HSO4-, OH-, CO32-, HCO3-

8/13/2019 2046Lab Advice


Insoluble Compounds: Ca (OH)2, CaSO4*2H2O, CaCO3, MgCO3 (white), Mg (OH)2,

Alums: Na[Al(H2O)6](SO4)2*6H2O, K[Al(H2O)6](SO4)2*6H2O, NH4[Al(H20)6](SO4) 2*H2O

2046lab advice

Documents