# Chemical Forums

## Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: DUDE778 on December 03, 2007, 12:58:52 AM

Title: Coulometric Titration (Controlled Current)
Post by: DUDE778 on December 03, 2007, 12:58:52 AM

60.00 mL of an acidic (0.1 M H2SO4) aqueous solution of phenol is titrated by bromine generated coulometrically from excess bromide (0.3 M KBr).

2Br−   -->  Br2 + 2e−

Under the experimental conditions the reaction proceeds only to monobromination.

C6H5OH + Br2 --> C6H4BrOH + HBr

At a constant generating current of 15.00 mA, the titration required 2094.9 s to reach the end point.
What was the molar concentration of phenol in the sample?

This makes no sense to me.
How do i use the current and what is 2094.9 s used for?

Title: Re: Coulometric Titration (Controlled Current)
Post by: Borek on December 03, 2007, 03:57:07 AM
Title: Re: Coulometric Titration (Controlled Current)
Post by: DUDE778 on December 03, 2007, 03:18:01 PM
m=1/F x QM/n
Q=I x t
i got m to equal 1.5317g x 1 mol / 94 g = 0.0163 mol
divided that by the volume 0.06L
i got 0.2715 M
Is that right?
Title: Re: Coulometric Titration (Controlled Current)
Post by: Borek on December 03, 2007, 04:42:22 PM
mA or A?
Title: Re: Coulometric Titration (Controlled Current)
Post by: DUDE778 on December 03, 2007, 05:32:34 PM
I used A.
Title: Re: Coulometric Titration (Controlled Current)
Post by: DUDE778 on December 03, 2007, 06:01:08 PM
I used
Q= 1.5A x 2094.9s
M= 94.0654
n=2
F=constand

Then i get
m=1.5317g
1.5317 g x 1 mol/94.0654 g = 0.0163 mol/0.060L
It says i got it wrong.   ???
Title: Re: Coulometric Titration (Controlled Current)
Post by: Borek on December 03, 2007, 06:02:02 PM
Why A if question tells mA?
Title: Re: Coulometric Titration (Controlled Current)
Post by: DUDE778 on December 03, 2007, 06:59:22 PM
I understand why i kept it as mA but this still only changes my answer by one decimal place.  i got 2.714 as my final answer.  Still says its wrong.
What am i doing wrong..   ???

Thank you for taking time to help me.  I appreciate it  :)
Title: Re: Coulometric Titration (Controlled Current)
Post by: Ipodlover on December 03, 2007, 08:05:19 PM
Yeah, you're doing this right man, i just did mine it worked fine. the problem is that you are supposed to used mA, not A, that is your number was 1.5mA, so divide 1.5/1000, that will be your amps.. USE THAT NUMBER, everything else seems fine.

Hey, im still working on this assignment, check the new postings cause im kinda confused as well for the previous ones....

My answer was X(E-3) M, just so you know if you're in the right range.

Later.
Title: Re: Coulometric Titration (Controlled Current)
Post by: Sev on December 03, 2007, 08:42:46 PM
Current is 15mA.  This is 15*10-3A (not 1.5A).

Quote
i got 0.2715 M
Is that right?

2 orders of magnitude greater than what I got  :)