[ghostanime2001]

Calculate the pH of a solution made by adding 1.000 mol of CH₃COONa and 0.500 mol of HCl to water and diluting to 1.000 L. K_A for CH₃COOH is 1.8 x 10^-5. Hint: if we mix the CH₃COO^- and H⁺ how much CH₃COOH do we produce ?

[Borek]

This is a pretty simple buffer question (direct application of the Henderson-Hasselbalch equation) and the hint hits the bullseye.

Please note you should show your attempts at solving the question to receive help. This is a forum policy.

[ghostanime2001]

I haven't learned that equation. How would you approach this problem without using that ?

[ghostanime2001]

Here is what my ICE table looks like

[Borek]

The hint you were given directly points to the use of the HH equation. But if you don't even want to try it, you can use your ICE table, it looks OK. It is just a longer route.

[ghostanime2001]

I got this question in grade 12 and im really sure i didn't learn that in that year.

[Borek]

Last time I checked knowing more than you were taught was considered an advantage, not a crime.

[ghostanime2001]

What is so wrong with learning alternate ways of doing something ? It would just enhance my understanding.

[fledarmus]

Well, if you can't use the Henderson-Hasselbalch equation because you've never seen it, and don't know what it is, what can you use? What do you know that would help you determine pH?

Do you know, for example, what the mathematical definition of pH is?

How about the definition of K_A?

What do you know about the concentrations of any of the materials in your solution?

[ghostanime2001]

I know that the initial concentrations of CH3COO- is 1M and H+ is 0.5M. What I don't understand is why the pH is 0.3 when it should be around 4-5.

[UG]

Notice the hint you were given, "Hint: if we mix the CH₃COO^- and H⁺ how much CH₃COOH do we produce ?". HCl is a strong acid and CH₃COO^- is a weak base, how much CH₃COOH will be formed? Further hint: you don't need an ICE table to work this out. Once you figure out this, it is a simple matter of substituting the concentrations into the K_a expression and then finding [H⁺]

[ghostanime2001]

I still don't understand

[Borek]

What I don't understand is why the pH is 0.3 when it should be around 4-5.

It is not 0.3, it is around 4-5. Not knowing where you got 0.3 from it is impossible to comment.

Show your work, it will help.

[ghostanime2001]

I got the answer to the first part, but not the second part. The second part says

What is the new pH if we add 0.020 mol of HCl to the solution in the previous problem ?

I have attached an ICE table for this second part. But again I get the wrong answer. Also, doing this for the first part gave me the correct answer except not this second part. I don't understand why it's giving me the wrong answer. I have understood Le Chatelier's principle. I know that when I add some more strong acid the weak base will be consumed and more acetic acid will be produced but the mathematical calculations do not seem to reflect this idea. Here is my attachment.

[ghostanime2001]

There are actually 3 parts to this whole question:

1. Calculate the pH of a solution made by adding 1.000 mol of CH₃COONa and 0.500 mol of HCl to water and diluting to 1.000 L. K_A for CH₃COOH is 1.8 x 10^-5. Hint: if we mix the CH₃COO^- and H⁺ how much CH₃COOH do we produce ? (Ans. 4.745)

2. What is the new pH if we add 0.020 mol of HCl to the solution in the previous problem (1.) (Ans. 4.780)

3. What is the new pH if we add 0.020 mol of NaOH to the solution in the previous problem (1.) (Ans. 4.710)

All these questions are done similarly except I do not understand why 2. and 3. are wrong. I have uploaded 3 pictures on ImageShack.

http://imageshack.us/photo/my-images/834/43048440.jpg/