Question

Hi everyone...I know how to do acid/base titrations but I am just having trouble writing a balanced equation and setting up Ka and Kb expressions from the word problems because I am not always sure how they break-up and fit back together! Here is an example and I need a general procedure to find the balanced equation and the Ka and Kb expression at all times: Here we go...10.0mL OF 0.1 m C6H5NH2 (Kb=3.8 x 10^-10) and 15.0 mL of 0.1 M HCL. Now I know HCL is a strong acid and C6H5NH2 is the weak base, but how do I write a balanced equation and an expression? Thanks!

Answer 1

sorry, that was supposed to be 10.0mL of 0.1 M C6H5NH2 (Kb=3.8 x 10^-10) and 15.0 mL of 0.1 M HCL

Answer 2

I am guessing that the equation is C6H5NH2 + HCL --> HC6H5 + NH2CL but I am totally not sure...maybe the HCL donates it's H+ to NH2 or something instead...please help

Answer 3

Could it balance out to be C6H5NH2 + HCL --> C6H5CL^- + NH3^+ ?

Answer 4

well the strong acid you know would completely dissociate
but the weak base would partly pick up a proton becoming NH3+, but there would be some left in the uncharged form (the extend of this dissociation can be found by its Kb)

Answer 5

SO IS THIS CORRECT THEN? C6H5NH2 + HCL --> C6H5CL^- + NH3^+

Answer 6

no i would say: C6H5NH2 + HCL --> C6H5NH3+ + Cl-

Answer 7

OH OK....i was just gonna say that too...
so then the NH3 stays attached to the c6hh5 ?

Answer 8

yeah, i would think so, since C6H5 would not be a stable molecule

Answer 9

so tell me if I have the logic correct then would you? a strong base or strong acid always completely dissociates but the weak base or weak acid will only change a little by accepting or giving up a proton?

Answer 10

yep thats right

Answer 11

they're called "strong" because they greatly contribute H+/OH- to the solution

Answer 12

so then something like ... HC6H5O2 + NaOH would become HC6H5O2 + NaOH --> NAC6H5O2 minus + H20 ?

Answer 13

OR WOULD IT BE ... HC6H5O2 + NaOH --> C6H5O2 minus + H20 + Na plus ?

Answer 14

i saw someone write it broken up into 3 pieces but it seemed weird to me...what do u think?

Answer 15

"OR WOULD IT BE ... HC6H5O2 + NaOH --> C6H5O2 minus + H20 + Na plus ?"

this one, Na+ is highly soluble in water so it will always (unless you really saturate it) be in its ionic form and thus soluble

Answer 16

wow...this gets confusing...so is it safe to say that when I see these problems, strong whatevers completely dissociate, weak whatevers will either grab or give one proton, and anything from column 1 and 2 from the periodic table will just float around in solution by itself in ionic form and i can just completely forget about it when I write the Ka and Kb expression?

Answer 17

yeah, that's right. i know, acid-base stuff is not the easiest stuff

Answer 18

so would the Kb expression for HC6H5O2 + NaOH --> C6H5O2 minus + H20 + Na be ... Kb = [C6H5O2minus] / [HC6H5O2][OH-] ?

Answer 19

sorry...flip those around

Answer 20

lemme try again...

Answer 21

well, in this case, you're using the acid in the equation, so you would write the reaction the opposite way if you're gonna use the Kb

Answer 22

HC6H5O2 + NaOH --> C6H5O2 minus + H20 + Na would be ...
Ka = [C6H5O2minus] / [HC6H5O2][OH-] ?

Answer 23

that would be for the Ka assuming I didn't go past equilibrium in the titration correct?

Answer 24

but for the Kb it would be...

Answer 25

HC6H5O2(aq) + NaOH(aq) --> C6H5O2 minus(aq) + H20(l) + Na(s) be ...
Kb = [C6H5O2minus] / [HC6H5O2][OH-] ?

Answer 26

sorry, my internet is crap. the Kb looks right, but the Ka, i don't think you'll use the OH-

Answer 27

like if you're starting the titration with pure base, and it's a strong base, you can assume the solution is reliant on the concentration of OH-.

Answer 28

if you're starting the other way around, with the weak acid, you assume the pH would be reliant on the concentration of H3O+ and the conjugate base

Answer 29

yah I think I see what you mean...soo...

Answer 30

soo.. if i'm starting without HC6H5O2, and i drop a little NaOH in there, there is no way the Ka expression should have OH- in the reactants side yet...not until equilibrium at least, which at that time, we would actually flip the equation around and be using a Kb instead? :o)

Answer 31

not without...i mean with

Answer 32

yep! thats exactly it

Answer 33

omg!... I actually sound like I know what im talking about! :o)~

Answer 34

don't share this around, my prof will kill me

Answer 35

can I like give you a medal somehow for this? i helped someone once and they gave me a medal but i don';t know how

Answer 36

it's basically a simplification for titrations (given that it follows the 5% rule), instead of going through simultaneous equations.

Answer 37

haha you click the blue button on the top right side of the boxes

Answer 38

done! you have now medaled in the academic olympics! yay! :o)

Answer 39

wanna hear something completely stupid?

Answer 40

haha wooo, well you pretty much got it yourself, i only guided ya

Answer 41

haha sure

Answer 42

my professor talks so fast, that i had to get a $125 digital recorder and when i re-listen to the lectures, i still can't understand her!

Answer 43

my final exam is on tuesday morning and i am sooo scared!

Answer 44

what? i have the opposite problem, my profs talk too slow i almost fall asleep OFTEN

Answer 45

why didn't you use your phone to record?

Answer 46

damnn, you still got time to review though

Answer 47

i start exams wed

Answer 48

because my phone's battery life is in dynamic equilibrium with the attention span of an ADHD patient on meth! :o)

Answer 49

see i used a chem term there in my funny! :o)

Answer 50

LOL

Answer 51

chem jokes FTW

Answer 52

this is the first time i've smiled all day! studying alone feels so bad...like nobody to depend on and the world is against u...ugh

Answer 53

have you heard this one?

a man walks into a bar, he orders some H2O.
another man walks into a bar, he orders some H2O too. (H2O2)

the second man dies

Answer 54

haha...nope havent heard that one...i like it!...i've got one too...

Answer 55

I've been studying all day too (its 4 am here) it's tiring, but hey, someones gotta do it.

Answer 56

What did Helium say to Hydrogen? He said "he he he he he"
hahah i like that one!

Answer 57

HAHA oh my sides

Answer 58

what did Sodium say to Calcium when when Calcium asked Sodium on a date?
Sodium said"Naaa" hehe

Answer 59

hahaha that sodium, he a player

Answer 60

fo sho! haha

Answer 61

well thatnk u sooo much for the help and the download...maybe u will be able to help me with more quetions in a bit if you are still on? thanks again Arron!

Answer 62

no problem at all! if I'm on and i can help, for sure. good luck with your studies!

Answer 63

thanks!.u 2