[toadesque]

4. Solutions of 0.05 M NaOH are added to 10.0 mL of 0.10 M 4-Chlorobenzoic acid (C7H5O2Cl). Calculate the pH for each of these solutions.




I don't understand how I use all these volumes of NaOH and for the acid to do this problem. They gave us the molarity but how do I incorporate the mL to get the pH's? Also, I need to get the pKa of the acid right? I've been working this problem for a long time but I can't seem to get it.

[spirochete]

This is the titration of a weak acid with a strong base.  First you need to look up the pKa for your acid, or the Ka.  pKa is the negative log of Ka, or Ka=10^-pKa.

NaOH is really just OH-.  Each molecule of OH- removes one H+ to form water, reducing the concentration of H+ and therefore raising the pH.  To find the moles of NaOH added, use dimensional analysis, eg:
 5.0*10^-3 L*0.05 mole NaOH = ?

Then you can figure out how much H+ you're starting with using the relationship Ka=([Conjugate base][H⁺])/[Acid]  You might remember your prof setting up a table in class that explains how to solve this algebraically for [H+]?  This will give your starting pH, remembering that pH=-Log[H+]


That should get you started

[toadesque]

Hm, I can't seem to find the pKa or the ka for Chlorobenzoic acid anywhere. I checked my book and online. Do you happen to know what it is or where i can find it?

thanks

[spirochete]

Also I forgot to mention that once you start deprotonating your acid with the strong base you have a mixture of acid and conjugate base.  You can then find the pH using the henderson hasselbach equation.

I can't find the pka either, try googling pka or ka tables.  If you're been assigned this problem it should be in your book.

[toadesque]

The thing is that my prof is a bit crazy. This is a take home test and he assigned us problems out of numerous books, one of them was a physical chemistry book haha. He said WE'RE supposed to calculate the pka because I remember some girl asked told him that it wasn't in the book and he said that's part of the question. But I have no idea how you're supposed to do that.

[spirochete]

Sorry I definitely don't know how to do this problem without a pKa or Ka for the acid. 

[toadesque]

I talked to my friend and he said the ka=1 x 10^-4

Don't know if that's correct but that's we he told me.

[spirochete]

http://www.zirchrom.com/organic.htm

 The pKa is 3.98 so you're friend is right.

edit:  need to stop multi tasking. . .