You need to prepare an acetate buffer of pH 5.79 from a 0.645 M acetic acid solution and a 2.53 M KOH solution. If you have 625 mL of the acetic acid solution, how many milliliters of the KOH solution do you need to add to make a bufer of pH 5.79? The pKa of acetic acid is 4.76.
the buffer equation
pH= pKa + log(A-/HA)
5.79 = 4.76 + log(ratio)
ratio = 10^(5.79-4.76) = 10.715
[A-] = 10.715*[HA]
assume:
initially:
[HA] = 0.645*625 = 403.125 mmol
[A-] = 0
after addition of:
KOH = MV = 2.53*Vbase
[HA] = 403.125 - 2.53*Vbase
[A-] = 2.53*Vbase
then
we know that
[A-] = 10.715*[HA]
2.53*Vbase = 10.715*( 403.125 - 2.53*Vbase)
solve for Vbase
2.53*Vbase = 10.715*( 403.125 - 2.53*Vbase)
2.53*Vbase =4319.484 - 27.1089*Vbase
Vbase ( 2.53 + 27.1089) = 4319.484
Vbase = (4319.484 )/( ( 2.53 + 27.1089) ) = 145.736
Vbase = 145.736mL of KOH is required
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