[axva1663]

Can someone help me out with this problem?

Aspirin has an acid dissociation constant Ka ~ 1 x 10^-4.

Diagram: C9H8O4 <--> C9H7O4- + H+

a) What is the pKa?
b) In what pH range is aspirin a good buffer?
c) When dissolved in the human stomach, a therapeutic dose of aspirin yields a 0.03 M solution. If the stomach contained only water   
   and the aspirin, what would be the concentration at equilibrium of aspirin (HA) and its conjugate base (A–), and the resulting
   pH?
d) The stomach actually contains about .02M HCl. Under these conditions, how much of the conjugate base would be formed? What is the
   pH?



This is what I have so far:

I assume that the "Diagram: C9H8O4 <--> C9H7O4- + H+" is equivalent to this: H2O + C9H8O4 <--> C9H7O4- + H3O+

a) pKa = -log(Ka) = 4
b) Henderson-Hasselbalch: pH = pKa + log([A-]/[HA]). Assuming [A-]/[HA] = 1, pH = 4.

And then I don't know how to do the others, since I don't know how to relate molarity to anything else.

Cheers,
axva1663

[plankk]

a) and b) are correct.

c) How looks a equation for Ka in this example? What is your starting concentration of aspirin acid? How it change during the dissociation and how much you will have it in the equilibrium point?

d) On what influences HCl?

[axva1663]

This all the information I was given. And that is why I am having trouble with it.

[plankk]

I understand. That questions is towarded to you.

[axva1663]

Oh...sorry. Well, the only equations concerning Ka that I know of are pKa = -log(Ka), Ka = [A-][H+]/[HA], Ka=Kw/Kb, and the Henderson-Hasselbalch equation. But how can I relate molarity to Ka? With the given molarity, I can only get a ration of moles per L of water.  Am I overlooking something?

[plankk]

Ka = [A-][H+]/[HA]

This one will be useful for us.

You know the value of Ka, but you don't know the rest. However, at the beginning in the solution where only aspirin (the concentration of it is c=0,03M). During the dissociation the concentration of it decreases, but in the same time the concentration of [H+] and [A-] increase. Use the symbol like x and say how amounts the concentration of [H+], [A-] and [HA] in the equlibrium.

[axva1663]

I am so retarded. Thank you so much for your help. It's been a while since I have done ICE tables and that sort of computation. Would you mind helping me in another problem?

[plankk]

Would you mind helping me in another problem?

If I can, why not. 

[axva1663]

OK, so it looks like I am still stuck on part d:

So I have the following reactions:

Cl- + Asp.    Aps.- + HCl

HCl + Asp.    Asp.- + Cl- + 2H+

I think the former is correct, but how would I use the ICE table to find [H+]?


The second problem is as follows:

The reaction of LiN(CHMe2)2 and acetone (Me2C=O) has a calculated Gibbs free energy of reaction of –23.8 kcal/mol. Assuming the pKa of the C–H bond in acetone is 20, what is the pKa of the conjugate acid of LiN(CHMe2)2?

Could you give me a hint as to where to start? Thank you so much.

[plankk]

Both reaction's equations are wrong. HCl is strong acid in comparison to aspirin acid, so the first one is impossible. What about second? If even HCl didn't dissociate completely, we would write two equations: one for aspirin and one for HCl. But HCl is as strong acid that we can omit not quite dissociation of it. In this case what is the beginning concentartion of H+?   

About second
Start with writing reaction: between acetone and LiN(CHMe2)2, between water and acetone and between water and conjugate acid of LiN(CHMe2)2. The first one can by made by the rest. Do you see that? If yes, you can use Hess's law.

One of equations of Gibbs free energy is connected with K. Do you know which one?