Naked Science Forum

Non Life Sciences => Chemistry => Topic started by: danica_aku on 16/05/2006 11:59:15

Title: How would I answer this question? [salts]
Post by: danica_aku on 16/05/2006 11:59:15

What is the pH of 0.100 M sodium phenolate, C6H5ONa, the sodium salt of phenol?[xx(][xx(][xx(]

Please help me...

Title: Re: How would I answer this question? [salts]
Post by: Cut Chemist on 16/05/2006 20:33:35

Henderson Hasselbach Equation-

pH = pKa - log ([HA] / [A-])

HA would be the OH form
and A- would be the O- form

0.100 mols/ liter of phenolate anion (O- form) from dissolving the salt

look up Pka of phenol

pH = -log [H+]

Title: Re: How would I answer this question? [salts]
Post by: danica_aku on 20/05/2006 06:20:59

tnx a lot![:)]

Title: Re: How would I answer this question? [salts]
Post by: DrDick on 25/05/2006 20:12:03

H-H doesn't work for this problem.  You need to be near the buffer region, where the phenoxide and phenol concentrations are within a couple orders of magnitude.

For this problem, you first need to calculate pKb of phenoxide, then Kb, then make an ICE table for the reaction:

    NaC6H5O + H2O <==> HC6H5O + NaOH
I    0.100                0       0
C      -x                +x      +x
-------------------------------------
E    0.100-x              x       x
     ~0.100

Kb = [HC6H5O][NaOH]/[NaC6H5O] = x^2/0.1
x = [OH-]
Once you find [OH-], you should be able to find pH pretty easily.

DrDick