# Chemistry Help (note somewhat advanced)

#### tintrail

##### Diabloii.Net Member

OK, well I'm in gen chem 2 in college, but since I placed out of the class, I only have to take the lab. It has been a good year and a half since I have done any inorganic chem, so I'm kinda rusty.

I have done the work, now all I am looking for is someone who is somewhat more advanced than myself to tell me if I am thinking along the right lines. If I am not, then I would appreciate a nudge in the right direction. :innocent:

k, here are the two questions.

1) We performed a complexometric titration, involving EDTA. The unknown solution contained H+, and so before we titrated the unknown solution with the EDTA (to find out the concentration of other ions in the solution [magnesuim and zinc]), we neutralized the H+ by adding a perdetermined amount of NaOH. Also, the solution was buffered to pH10, so that the Mg and Zn would form the complex more easily.
The question is, why was the acid neutralized?

Answer: if the acid were not neutralized, then the pH would have been significantly lower, which means that the complex may not have been able to form.

2) The capacity of an ion exchange resin is usually expressed on the basis of the number of 'milliequivalents' of charge that can be exchanged per mL of the resin bed. If a packed column with a capacity of 3.5 milliequivalents/mL of resin has a total resin volume of 15.5mL, what is the maximum number of millimoles of Ca2+ that could be exchanged before the resin is saturated?

(3.5 * 15.5) / 2 = 27.125 millimoles of Ca2+.

The
(3.5*15.5)

gives you the total amount of charge that can be stored, and then you divide by 2, because each millimole of Ca2+ has a charge of 2.

Does this sound right?
Thanks for your help if you can indeed help. peace :teeth:

EDIT: ok, ill just go finish my differential equations homework, read the italian play, and then come back and see whats going on... what i would give for it to be friday, there to be a bottle of captain morgans rum by my side, and a party with hot girls to go to... ahhhhhhhhhhhh
.

#### ZeppelinAngel

##### Diabloii.Net Member
say hello to my little friend...

#### AeroJonesy

##### Diabloii.Net Member
When tintrail's question gets answered, I'm going to need some help with my linear control systems engineering. Or perhaps my aerodynamics classes...

#### Jigga-Scrooge

##### Diabloii.Net Member
im scared. i think high school chemistry is hard.

#### Module88

##### Diabloii.Net Member
Jigga-Scrooge said:
im scared. i think high school chemistry is hard.
AP or CP? CP is cake. AP is hell.

#### Wuhan_Clan

##### Diabloii.Net Member
AeroJonesy said:
When tintrail's question gets answered, I'm going to need some help with my linear control systems engineering. Or perhaps my aerodynamics classes...
You doing aerospace? I am. Why is my low speed aerodynamics book completely written in calculus? The cover looked so cool, but when I open it up, its just lines and lines and lines ...... *gets dizzy and falls asleep*

#### tintrail

##### Diabloii.Net Member
Wuhan_Clan said:
You doing aerospace? I am. Why is my low speed aerodynamics book completely written in calculus? The cover looked so cool, but when I open it up, its just lines and lines and lines ...... *gets dizzy and falls asleep*
k people.. i appreciate your enthusiasm, but when i come back and see that there are replies i get somewhat happy, only to find out it is people remarking on the difficulty of the subject material etc.. doesnt help, so plz try not to spam the thread

#### AeroJonesy

##### Diabloii.Net Member
Sorry tintrail, I was more making the comment that you probably know more chemistry than 99.99% of the people here so you might not be able to find much help. Unless Mixed shows up. Besides, the more we talk, the higher up the thread stays and the better chance you have of someone who can actually help you seeing it.

Wuhan, yeah, I'm in my third year of aerospace, and more than halfway through! Are you talking about the Anderson book that's called "Introduction to Aerodynamics" or something like that?

#### tintrail

##### Diabloii.Net Member
AeroJonesy said:
Sorry tintrail, I was more making the comment that you probably know more chemistry than 99.99% of the people here so you might not be able to find much help. Unless Mixed shows up. Besides, the more we talk, the higher up the thread stays and the better chance you have of someone who can actually help you seeing it.
yeah i thought of this.. thanks man.. sorry i seemed pissy.. =).. just dissapointed that noone can help, and mixed isnt around...

anywho, i emailed my prof, so i guess i should be alright...

#### Anyee

##### Diabloii.Net Member
1) Okay, here's what Google tells me, from this pdf. It seems that the maximal binding of Mg (and Zn, but that's from another article) is at a pH of 10. The Mg would probably dissociate from the EDTA at a pH lower than that, meaning that your titration would probably take longer and be less accurate. Interestingly, you use a pH of 8.0 in most biological systems involving EDTA.

2) You're missing a step, I think.
You've gone from milliequivalents/mL to just milliequivalents. You need to deal with "per mole" somewhere in there.

Edit: I disagree about dividing by 2. I think you should be multiplying, since each Ca ion can hold 2 charges. I need to examine the problem more.

#### tintrail

##### Diabloii.Net Member
Anyee said:
1) Okay, here's what Google tells me, from this pdf. It seems that the maximal binding of Mg (and Zn, but that's from another article) is at a pH of 10. The Mg would probably dissociate from the EDTA at a pH lower than that, meaning that your titration would probably take longer and be less accurate. Interestingly, you use a pH of 8.0 in most biological systems involving EDTA.

2) You're missing a step, I think.
You've gone from milliequivalents/mL to just milliequivalents. You need to deal with "per mole" somewhere in there.

Edit: I disagree about dividing by 2. I think you should be multiplying, since each Ca ion can hold 2 charges. I need to examine the problem more.
sweetness, thanks anyee

and in regards to #2, i forgot to add this little bit, so i think im ok..:

the question should read 'milliequivalents or millimoles'... not just milliequivalents. i typed the question up in a hurry. so i think the dimensional analysis works now.

once again, thanks =).. i can rest easy

----------------------------------------
say hello to my little friend.... :teeth:

#### Anyee

##### Diabloii.Net Member
Don't forget to reference the article in your discussion, else it is plagiarism.

You're welcome.

#### ASTRALdragon

##### Diabloii.Net Member
Anyee said:
1) Okay, here's what Google tells me, from this pdf. It seems that the maximal binding of Mg (and Zn, but that's from another article) is at a pH of 10. The Mg would probably dissociate from the EDTA at a pH lower than that, meaning that your titration would probably take longer and be less accurate. Interestingly, you use a pH of 8.0 in most biological systems involving EDTA.

2) You're missing a step, I think.
You've gone from milliequivalents/mL to just milliequivalents. You need to deal with "per mole" somewhere in there.

Edit: I disagree about dividing by 2. I think you should be multiplying, since each Ca ion can hold 2 charges. I need to examine the problem more.
noooooooo she beat me to it!! after examining my 2-3 year old notes from general chem 2, it is what she says it is. i vaguely remember this experiment. this was a loooong time ago for me. i also remember that i owed the university \$80 for breaking a pipet :grrr: . later on you guys will identify unknown solids which is actually pretty fun. when you get to organic chem, identifying them are not so fun but you get to work with sodium fusion which is ULTRA kool. oh yea, a word of caution, tread carefully when you get to kinetics in your class. it took me awhile to grasp that. good luck.

#### tintrail

##### Diabloii.Net Member
Hey, I just wanted to say thanks to everyone who helped yesterday. Special shoutouts go to Anyee, Astral and yes, Aero for keeping this thread at the top so people could view it.

I emailed my professor yesterday as well, and he replied that my answers were correct =). :teeth:

Anyways, Astral, we were doing more titrations today, and we analysed the amount of Cl in our urine =).

We had the option of using our own samples or one provided by the lab. I cannot for the life of me think why someone would choose the one given by the lab. :surprise:

Anyways, titrations are cool, although i need to be slightly more careful when watching for the endpoints. I tend to go too slowly, then get impatient and speed it up right at the equivalence point, and in doing so slightly overshoot the end.

Anyways. Thanks again guys. :howdy:

##### Diabloii.Net Member
The answer is c. It's always c.

#### Corneo

##### Diabloii.Net Member
tintrail said:
Hey, I just wanted to say thanks to everyone who helped yesterday. Special shoutouts go to Anyee, Astral and yes, Aero for keeping this thread at the top so people could view it.

I emailed my professor yesterday as well, and he replied that my answers were correct =). :teeth:

Anyways, Astral, we were doing more titrations today, and we analysed the amount of Cl in our urine =).

We had the option of using our own samples or one provided by the lab. I cannot for the life of me think why someone would choose the one given by the lab. :surprise:

Anyways, titrations are cool, although i need to be slightly more careful when watching for the endpoints. I tend to go too slowly, then get impatient and speed it up right at the equivalence point, and in doing so slightly overshoot the end.

Anyways. Thanks again guys. :howdy:
I had to carry out a titration experiment a couple days ago, did you suck up some of the liquid into a pipet aka "titration theif"? In case you over shoot, you still have some of that liquid to return it back to normal color.

#### tintrail

##### Diabloii.Net Member
Corneo said:
I had to carry out a titration experiment a couple days ago, did you suck up some of the liquid into a pipet aka "titration theif"? In case you over shoot, you still have some of that liquid to return it back to normal color.
Sounds like a good idea, not sure whether I am actually allowed to do something like that... I might ask my professor. Thanks for the tip. :teeth:

#### Corneo

##### Diabloii.Net Member
Yes, this method was written in my lab manual. Once you see the color indicator change, stop the titration, add some of the sucked up liquid you reserved (approx 1ml). The solution should change color back and then drop by drop you add the base again.

#### Jigga-Scrooge

##### Diabloii.Net Member
The answer is c. It's always c.

i once took a test where some of the answers were a, b, d, and e.

#### MixedVariety

##### Banned
You done good, Tin, and everyone who helped out. Sorry I didn't see this until just now, but I can see I was not needed anyway...sniff...