[eniko]

Hi!
Could anyone please help me?! I have to calculate the ionic strength of an acetate buffer of pH 4.6, 0.1 M  prepared from 99.5% acetic acid and solid sodium hydroxide.
According to Henderson-Hasselbach equation the preparation needs 0.14 M acetic acid and 0.1 M acetate ion.
I=1/2([acetate ion]*(acetate ion charge)^2+[sodium ion]*(sodium ion charge)^2)=1/2(0.1*1^2+0.1^2)=0.1
Is this correct?

[Borek]

According to Henderson-Hasselbach equation the preparation needs 0.14 M acetic acid and 0.1 M acetate ion.

Doesn't look right to me, but perhaps I am misunderstanding what you wrote. 0.1 M acetate buffer is 0.1 M in sum of acetic acid and acetate anion, while the one you suggest is 0.24 M.

[eniko]

0.24 M of acetic acid in total.

[Babcock_Hall]

If someone says to me that he or she made a 0.1 M acetate buffer, I interpret that to mean that the sum of all forms of acetate (conjugate base plus conjugate acid) is 0.1 M.

[eniko]

The recipe said that we need an acetate buffer pH 4.6: c(acetic acid)=0.1 mol/L + c(sodium acetate)= 0.1 mol/L.
We wanted to prepare 1L of solution from 99.5 % acetic acid (molecular weight: 60 mol/kg, density: 1.05 g/mL, pKa1=4.756) and solid sodium hydroxide (molecular weight 40 g).
So, we first calculated the ratio acetic acid - acetate ion using the Henderson-Hasselbach equation, as it follows:
pH=pKa+log([acetate]/acetic acid])
4.6=4.756+log([acetate]/acetic acid])
0.144=log([acetate]/acetic acid])
1.39=[acetate]/acetic acid]
[acetate]=1.39*[acetic acid]
Thus, we needed in total 2.39 M acetic acid and 1 M of NaOH. Because we needed a buffer of 0.1 M, keeping the obtained ratio, we obtained 0.239 M acetic acid and 0.1 M NaOH.

0.239 mol *60 (molecular mass)=14.34 g / 1.05 (density) = 13.7 mL acetic acid in 1000 mL
0.1 mol NaOH*40 (molecular mass)= 4 g of NaOH in 1000 mL solution.

Thus, 1L of 0.1 M buffer solution was prepared with 13.7 mL acetic acid and 4 g of solid NaOH.
For this solution I have to calculate the ionic strength.

[Borek]

You were told twice you are wrong, and 0.1 M acetate buffer doesn't mean a buffer in which acetate is at 0.1 M. No matter how many times you will explain to us details of your work it won't make it correct.

What you have is a 0.24 M acetate buffer, not 0.1 M acetate buffer.

c(acetic acid)=0.1 mol/L + c(sodium acetate)= 0.1 mol/L.

Not sure what you mean by that, but no way it works

c(acetic acid) + c(sodium acetate)= 0.1 mol/L

would be a correct definition of 0.1 M acetate buffer (and your concentrations don't fit)

c(acetic acid)=0.1 mol/L and c(sodium acetate)= 0.1 mol/L

would be a 0.2 M acetate buffer with pH 4.75, not 4.6.

Correct approach to buffer calculation in this case is described here: http://www.chembuddy.com/?left=buffers&right=composition-calculation (see the last problem, bottom of the page).