[b323]

If a solution is created by dissolving 1.935 g of sodium acetate in 850 mL of 0.405 M acetic acid at 25 degrees C, what is the concentration of the acetate ion in the final solution? Assume that the volume of the solution does not change. Ka for acetic acid is 1.8*10^-5 at 25 degrees C.

[rabolisk]

Please attempt the problem first.

[b323]

I tried to find the moles of odium acetate, which turned out to be 0.02359 and the moles of acetic acid which is 0.34425. Then I did an "Ice" table with acetic acid going into H+ and the acetate ion, using Ka to find the concentration of the acetate ion and the hydronium ion.  I found them to both be 0.0027 M. From there I do not know what to do though.

[Borek]

Henderson-Hasselbalch equation.

[rabolisk]

Then I did an "Ice" table with acetic acid going into H+ and the acetate ion, using Ka to find the concentration of the acetate ion and the hydronium ion. I found them to both be 0.0027 M.

This is not right. Remember that the dissociation of acetic acid will be affected by the acetate that is already present in solution. Once you get the (correct) value for the amount of acetate, all you have to do is divide that by the volume to get its concentration.

HH equation is actually unnecessary because the question does not ask for pH.

[b323]

So do I need to find the amount of acetate that comes from the sodium acetate and the amount that comes from the acetic acid, add the number of moles from each then divide by volume? How do I go about doing this without the HH equation? I first thought that a balance equation of sodium acetate and acetic acid was necessary, but I was unable to come up with one for this solution.

[rabolisk]

Do you understand how to calculate with the common ion effect? You are basically doing calculation for dissociation of acetic acid, but the initial concentration of acetate is not 0.

[Borek]

HH equation is actually unnecessary because the question does not ask for pH.

My bad, shouldn't answer too fast.

[Shelly A. Henry]

It makes my head rock and roll. i will try to figure out on my own answers. Hahaha!

[AWK]

If a solution is created by dissolving 1.935 g of sodium acetate in 850 mL of 0.405 M acetic acid.
What is the concentration of the acetate ion in the final solution? Assume that the volume of the solution does not change

Neither temperature nor Ka are neded for solution of this simple stoichiometric problem.
just convert mass of acetate into moles and then use definotion of the molar concentration (moles per volume in Liters)

[Borek]

Neither temperature nor Ka are neded for solution of this simple stoichiometric problem.
just convert mass of acetate into moles and then use definotion of the molar concentration (moles per volume in Liters)

You have fallen into the same trap I did.

Not that the difference is large, less than 1%, so it makes me wonder what the intended answer to the question is.

[AWK]

Neither temperature nor Ka are neded for solution of this simple stoichiometric problem.
just convert mass of acetate into moles and then use definition of the molar concentration (moles per volume in Liters)

You have fallen into the same trap I did.

Not that the difference is large, less than 1%, so it makes me wonder what the intended answer to the question is.

If a solution is created by dissolving 1.935 g of sodium acetate in 850 mL of 0.405 M acetic acid at 25 degrees C, what is the concentration of the acetate ion in the final solution?

.
There is no pH calculation as title "Buffers" suggests. May be you can check this problem with your software.
The large excess of acetic acid (about 20 times) is large enough to neglect the hydrolysis of acetate anion.

[Borek]

There is no pH calculation as title "Buffers" suggests. May be you can check this problem with your software.

I did before posting.

The large excess of acetic acid (about 20 times) is large enough to neglect the hydrolysis of acetate anion.

At equilibrium there is 0.02775M of Na⁺ and 0.02812M of acetate in the solution - concentration of acetate is about 1% higher than what you put into solution. While assumption that concentration is just a stoichiometric one is not bad, it doesn't give exact result.