Finding the concentration of a weak acid in a weak acid/strong base titration

[eric343]

So I'm doing this lab report...

We did a titration of NaOH and vinegar (acetic acid; CH3COOH). As it turned out, the equivalence point happened when 14.175mL of NaOH was added to the solution of vinegar ("unknown" concentration).

Now, acetic acid is a weak acid, which means I can't simply calculate the number of moles of NaOH used and say that's the number of moles of acid, and divide that by the initial volume of acid solution.

How do I go about finding the initial concentration of acetic acid?

 

[psxguy85]

Quote:

Originally Posted by eric343 So I'm doing this lab report... We did a titration of NaOH and vinegar (acetic acid; CH3COOH). As it turned out, the equivalence point happened when 14.175mL of NaOH was added to the solution of vinegar ("unknown" concentration). Now, acetic acid is a weak acid, which means I can't simply calculate the number of moles of NaOH used and say that's the number of moles of acid, and divide that by the initial volume of acid solution. How do I go about finding the initial concentration of acetic acid?

What molar NaOH are you using in this experiment, and what volume of vinegar are you using.

 

[eric343]

Quote:

Originally Posted by psxguy85 What molar NaOH are you using in this experiment, and what volume of vinegar are you using.

0.100M NaOH, 10mL vinegar. (then added 50mL of water to have a sufficient volume of solution to immerse the pH sensor in)

 

[pspivak]

Try N1V1=N2V2

 

[eric343]

Eh?

 

[MuZI]

I remember doing this titration lab in Chem, you`re question is a bit werid, post the result table?

Ok well i have the notes from chem, are you using only titration or are you using PH curves also?

Do you know how to do stoichiometery well?

 

[eric343]

Quote:

Originally Posted by MuZI I remember doing this titration lab in Chem, you`re question is a bit werid, post the result table? Ok well i have the notes from chem, are you using only titration or are you using PH curves also? Do you know how to do stoichiometery well?

I can do stoichiometry.

We did the titration using a pH probe in the solution and using LoggerPro to generate a result table and a pH curve of pH vs. volume of NaOH added...

I was under the impression that the result table was only needed to determine the equivalency point?

 

[MuZI]

Right i just wanted to look at it so everything was there for me, you mentioned weak acid?

Weak Acid...Proton-Transfer Reaction

Are you in a Honors/AP class?...I hope not..

 

[eric343]

Quote:

Originally Posted by MuZI Right i just wanted to look at it so everything was there for me, you mentioned weak acid? Weak Acid...Proton-Transfer Reaction Are you in a Honors/AP class?...I hope not..

Yes... Acetic acid is a weak acid.

It's an honors class... unfortunately, I somehow missed the explanation of how to do this -- my notes leave out all the vital details, as if I'd fallen asleep and hadn't noticed it

I'm going to ask about it tomorrow.

 

[MuZI]

Write the Chemical equation that describes the neutralization of hydrochloric acid with sodium hydroxide.

Write the sample stoichiometeric calculation for taking the known data and calculating the conectration of the acid. The concentration of the base should be given...

I Completely forgot the equation, all i can say is it was a pain and long..i should have kept my labs..

 

[eric343]

Finding [HCl] is easy -- it's a strong acid. (and that was the first part of the lab)

But CH3COOH is a weak acid, with a Ka and all that...

 

[MuZI]

I was in a General class but i think i know how to do this, it would be much easier if i had my old labs.

This is a long shot but you might have to do the equation reversing the Proton Transfer, use THAT to get the moles because it would take into account that the acid is weak...

It is getting late, don't mind me if that made no sence..are you in summer school?

Edit: In a proton transfer the Acid gives a H+ proton to the base, ok, i don`t think that would help you, i'm sorry...

And the mole bridge wouldn't work?

 

[dffman2001]

ok, its relatively simple

CH3COOH + NaOH -> H20 +NaCH3COO

0.014175 L x 0.1mol/L x 1/1 x 1/0.01 L

volume of NaOH x concentration of NaOH x molar ratio x volume of vinegar

so the concentration of the vinegar is 0.147175 mol/L

if i did this correctly.

hope this helps!

 

[MuZI]

Sounds about right, you did the mole bridge.

So the molality = 0.147175 mol/L

But that weak acid still gets to me..did that affect the reaction?

CH3COOH + NaOH -> H20 +NaCH3COO

Edit: Are you sure that reaction is right? looks awkward..

 

[dffman2001]

well, since you are bringing it to its equivalence point, it dosen't matter if it is a weak acid or not, since all the H3O+ will react once the CH3COOH ionizes, therefore Le Chatalier's principle will state that the equilibrium will drive towards the H3O+ until all the CH3COOH is consumed - which is at the equivalence point

the equivalence point should be greater than 7, as the conjugate base of vinegar is a base. thats all that changes in a reaction between a strong base and a weak acid.