[evilamoeba]

 

hey all, i'm confused!
Here's the question:

1.37g of sodium acetate is dissolved in H20 and the volume is made up to 250mL in a volumetric flask. Whats the pH of the resultant solution? (Ka for acetic acid = 1.75 x 10-5)


My working:

no. Moles = mass/molecular weight
=1.37/82
=0.0167

sodium acetate -> (250 x 0.0167)/1000
=0.004176

[H+] = Ka (acid/base)
        = 1.75 x10E-5 (0.004176)
        = 0.000,000,073

- log (0.000,000,073)

= 7.136

I think this is wrong because i thought the solution would be more basic. (around pH of

Please, any help would be GREATLY APPRECIATED, for instance if you can see an error in my working.



Thank you.
Evilamoeba

[Borek]

sodium acetate -> (250 x 0.0167)/1000
=0.004176

I suppose it is just a mistake, but still - why do you multiply number of moles by volume? What is definition of molar concentration?

[H+] = Ka (acid/base)

So far so good, although this equation is of no use here.

= 1.75 x10E-5 (0.004176)

Could you please indicate acid and base that you put into the equation?

TBH with you - whole approach is wrong. This is in fact weak base solution. Identify this base, find Kb and go from there.

[english]

Write a chemical equilibrium first.  Answers a lot of questions.

Acetate will react with water to form a small amount of HO^- and AcOH (acetic acid).

[HO] gives you a direct pOH calculation, from which you can get pH.  AcOH will contribute to the pH as well, but the amount is so tiny it will probably not matter.  To be safe, figure out how much H⁺ dissociates from AcOH as well (write an ICE table).  If the order of magnitude is less than -5, you can probably ignore it.

Hint:  AcO^- + H₂O ---> AcOH + HO^-

then from the above equilibrium, we can write an additional dissociation of AcOH in water (water is in excess; there's plenty of it):

AcOH + H₂O ---> H⁺(or H₃O⁺) + AcO^-

The ions that you need to be worried about are in bold.