Buffers Lab pH assigned: 5 acid/base pair: acetic acid solution (vinegar) and so

Question

Buffers Lab

pH assigned: 5

acid/base pair: acetic acid solution (vinegar) and sodium acetate; Ka of HC2H3O2 is 1.8 x 10-5

3. Use the Ka equilibrium or the Henderson-Hasselbalch equation to determine the molarity of the conjugate acid and the molarity of the conjugate base needed to make 25 mL of your buffer. Shoot for about a 0.1 M concentration of the acid in your buffer pair, and adjust the base as necessary to attain your assigned pH.

(Additional directions for step 3.)

Once you have concentrations for the acid and base in your buffer, calculate the masses or volumes of each needed to make 25 mL of buffer solution. Both will be dissolved in the same 25 mL. Solids available are: KH2PO4, K2HPO4, KHCO3, and K2CO3, and NaC2H3O2·3 H2O. HC2H3O2 is a caustic liquid, so it will be available as a 1.00 M stock solution rather than the pure compound.

4. You must write out a procedure for making 25.0 mL of your buffer.

Please show all work

Explanation / Answer

V = 25 mL buffer

pKa = -log(Ka) = -log(1.8*10^-5) = 4.75

[Acid] = 0.1 M

so.. apply Hendesron Hasselbach equations

pH = pKa + log(Acetate/Acetic Acid)

5.0 = 4.75 + log([Acetate]/0.1)

[Acetate] = 0.1*10^(5-4.75)

[Acetate] = 0.177827 M will be neded

From the list you presented

ONLY NaC2H3O2·3 H2O will help

NaC2H3O2·3 H2O --> NaC2H3O2(aq) + 3H2O(l)

NaC2H3O2 --> Na+(aq) + C2H3O2-(aq)

Note that you are stating 0.1 M of acetic acid, but it is only available at 1 M so it will be diltued as well

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