Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: huskywolf on November 06, 2010, 11:59:30 AM

Title: pH at equivalence point
Post by: huskywolf on November 06, 2010, 11:59:30 AM

Hello.

I done an experiment where I titrated NaOH into CH₃COOH.

I started by putting 0.025L of 0.1M CH₃COOH into a beaker, I then added 0.025L of deionized water to this, the initial reading of pH was 3.27. I added NaOH until the pH was 12(0.046L NaOH ) in total.

I want to work out what the pH is at the equivalence point.

Can you tell me how to do this please?
is CH₃COOH M0.05 now as 0.025L of water diluted it?
I have this done so far....
n(CH₃COOH)=0.025 x 0.05/1=1.25x10^-3mol
and
n(OH^-)= 0.046 x 0.1/1= 4.6 x 10^-3 mol

How can I calculate the pH at the equivalence point please?
Thanks

Title: Re: pH at equivalence point
Post by: Borek on November 06, 2010, 12:09:23 PM

http://www.titrations.info/acid-base-titration-equivalence-point-calculation

Title: Re: pH at equivalence point
Post by: huskywolf on November 06, 2010, 12:15:07 PM

Quote from: huskywolf on November 06, 2010, 11:59:30 AM

Hello.

I done an experiment where I titrated NaOH into CH₃COOH.

I started by putting 0.025L of 0.1M CH₃COOH into a beaker, I then added 0.025L of deionized water to this, the initial reading of pH was 3.27. I added NaOH until the pH was 12(0.046L NaOH ) in total.

I want to work out what the pH is at the equivalence point.

Can you tell me how to do this please?
is CH₃COOH M0.05 now as 0.025L of water diluted it?
I have this done so far....
n(CH₃COOH)=0.05 x 0.05/1=2.5 x 10^-3mol
and
n(OH^-)= 0.046 x 0.1/1= 4.6 x 10^-3 mol

How can I calculate the pH at the equivalence point please?
Thanks

Title: Re: pH at equivalence point
Post by: rackye on November 06, 2010, 12:19:31 PM

Remember this: acetic acid is a weak acid so is ruled by an equilibrium constant (ka) the value of Ka determine the amount of protons in the solution. with water you only dilute the acid, (protons contribution of the water are too small to be consider) the other thing to remember is that in the equivalence point moles of H+ at the same of OH- so if you used an indicator and you have the volume of NaOH at the change of color you can easy calculate the PH.

Title: Re: pH at equivalence point
Post by: huskywolf on November 06, 2010, 12:29:32 PM

Quote from: Borek on November 06, 2010, 12:09:23 PM

http://www.titrations.info/acid-base-titration-equivalence-point-calculation

I dont understand much of those calculations , can you explain how to use my information to calculte the pH at equivalence point please

Title: Re: pH at equivalence point
Post by: huskywolf on November 06, 2010, 12:58:49 PM

I have worked out that the pH at the end of the reaction is 12.3, I know that OH^- is in excess of 0.0021mol after the two react.

but how do I get the pH at equivalence point? do I take 12.3 and minus 3.27(the initial pH of weak acid?)

Title: Re: pH at equivalence point
Post by: huskywolf on November 06, 2010, 03:10:26 PM

I solved it .  :)