[Panzer22]

So, we have 25ml of Acetic acid, concentration unknown, with 3 drops of phenolphtaleine as an indicator. We are adding droplets of NaOH, c = 0.1 mol/L, 2 mL at a time, to find out at what point the solution goes from clear to pink (showing the complete neutralisation of the acetic acid. I have a few questions with regards to this:

CH₃COOH + H₂O  <-->  H₃O⁺ + CH₃COO^-

1. How to calculate the concentration of the CH₃COOH?
I think we need to use the formula : c1v1 = c2v2, using 0.1 mol/L as c1; 0,031L (volume of NaOH added in total) as v1; and 0,025L as v2 (volume of acetic acid, henceforth referred to as HAc). This gives [HAc] of 0,124 mol/L. Does that look right?

2. We mesured a pH of approximately 4 at the beginning. Using this value, we found [H₃O⁺] to be 1 x 10^-4 mol/L. How do we use this value to calculate the [CH₃COO^-]?

3. Using all of these values, is there a way to calculate K_a? I know it's supposed to look like [H₃O⁺][CH₃COO^-] / [CH₃COOH], but am i using the correct values? The real value of the constant is 1,8 x 10^-5, but I'm way off, so i must be doing something wrong. Do I have to use the IVE table?

Any help is appreciated,

Spencer

[AWK]

1. OK
2 .

We mesured a pH of approximately 4 at the beginning.

Does it mean you measured pH of acid?
Then conceetration of CH3COO- is equal to that of H3O+

For your concentration of acetic acid, pH should be about 2.8,
hence a big error ok Ka.

[Panzer22]

Yes, we mesured the pH of the acetic acid before beginning to neutralize it. We got a reading of 4. You said this means the [H3O+] and [CH3COO-] are equal. This means both of them are equal to 1 x 10^-4 mol/L?

And the pH should be 2.8? How did you calculate that? With what numbers? Furthermore, how can I get a Ka that falls within reason?

Thanks,

Spencer

[AWK]

Since degree of dissociation is small [H3O+]/c(CH3COOH) you can assume that an equlibrium concentration of acetic acid is the same as the total concentration and you have all needed numbers for calculations. The other approximation of these calculations is neglecting of concentration of H3O+ from dissociation of water (it is of order 10

-7

)