Question

# You need to prepare an acetate buffer of pH 6.12 from a 0.755 M acetic acid...

You need to prepare an acetate buffer of pH 6.12 from a 0.755 M acetic acid solution and a 2.43 M KOH solution. If you have 475 mL of the acetic acid solution, how many milliliters of the KOH solution do you need to add to make a buffer of pH 6.12? The pKa of acetic acid is 4.76.

Acetic Acid,being a weak acid, dissociates as-

Hence the Acid dissociation constant,

where the concentrations are the euilibrium concns [eqn 1]

Now and

Rearranging eqn 1,

pH of the Buffer=6.12,Hence in the Buffer= mol/L

The above is the required Acetic acid to acetate concn ratio required to get the pH 4.76.

Now to obtain the Acetate ions,we add KOH to the acid which reacts as following-

From the balanced eqn,it is clear that

No of equivalents of KOH added=No. of equivalents Acetate ion is produced.

As we know that 2.43M KOH soln is used,so if 'V'L of KOH is used,we can give the equation-

where n is the no of moles.

Now the no of moles of Acetic Acid used=

So,equilibrium no of moles of Acetic Acid=0.358625-2.43V

.

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