[Halogen876]

I have a textbook question that I am trying to figure out (this is not homework, it is general interest) that I am having trouble with. Here is the question:

What is the pH of a solution formed by addition of 30.0mL of 0.200F HCl to 25.0mL of 0.100F disodium maleate?

Here is what I have tried:
   moles of HCl = 0.00600mol
   moles of maleate = 0.00250mol

0.00600mol-0.00250mol=0.00350mol hydronium left over
0.00250mol of hydrogen maleate formed

0.00350mol-0.00250mol=0.00100mol hydronium left over = final moles of hydronium
0.00250mol=mol of maleic acid

final concentration of hydronium = 0.00100mol/0.0550L = 0.0182mol/L
so pH is the negative log of that so pH = 1.74?

That is what I am getting and it makes sense to me, but the answer in the book is 1.51. I'm wondering if there is something I am missing or is the answer in the book wrong?

Any help would be greatly appreciated!

Thanks!

[Borek]

Some of the maleic acid dissociates. Use ICE table just for the first dissociation step and see if it works.

[Halogen876]

Thanks for your suggestion. I tried setting up an ICE table taking into account the leftover hydronium and then accounting for some of the maleic acid dissociating.

When I filled in the ICE table, my equilibrium line is:
HOOCCH=CHCOOH  +  H2O     HOOCCH=CHCOO- +  H3O+
0.00250-x                     -                                  x                 0.00100+x

When I plug that into the quadratic, I end up getting x=0.00201mol
[H3O+] = (0.00100+0.00201)mol/(0.055L) = 0.0547mol/L
pH=-log(0.0547)=1.26

So I'm still missing something since the answer is supposed to be 1.51..

Any more help would be greatly appreciated!

[Borek]

Why do use numbers of moles (and not concentrations) in ICE table?

[Halogen876]

The way I was taught ICE tables was that you could use either moles or concentrations - just to remember that if you use moles to remember to convert to concentrations at the end...this method has always worked for me before so I don't see a reason why it wouldn't work this time, but if there is some reason why my method of using moles isn't good for this problem and someone could let me know, that would be greatly appreciated!

[Borek]

The way I was taught ICE tables was that you could use either moles or concentrations

That's wrong. It works only sometimes. ICE table is just a convenient way of keeping track of the stoichiometry. Once you know stoichiometry you plug it into

[tex]K_a = \frac {[H^+][A^-]}{[HA]}[/tex]

which uses concentrations, not number of moles.

Now, concentration is

[tex]C = \frac n V[/tex]

So sometimes plugging numbers of moles will work (when Ka is unitless, and all volumes cancel out) - but in general it will not.

[Halogen876]

That worked! I plugged the concentrations into the ICE table and came up with the correct answer. Thank you so much for all your *delete me*

[Halogen876]

Don't know what happened there when I posted that reply but it was supposed to say thank you so much for all your help!