A buffer consists of a weak acid or base, and the salt of that acid or base. Con
ID: 588988 • Letter: A
Question
A buffer consists of a weak acid or base, and the salt of that acid or base.
Consider a buffer made by combining the weak acid HA with its salt, A:
HA(aq)+H2O(l)A(aq)+H3O+(aq)
Ka=[A][H3O+][HA]
The H3O+ concentration of this buffer can be calculated using the Ka expression:
[H3O+]=Ka[HA][A]
You can use this expression when solving for pH, or you can simplify the expression even more. Taking the negative log of this expression, you have
log[H3O+]=log[Ka]log[HA][A]
Since log[H3O+]=pH and log[Ka]=pKa, this can be further simplified to
pH=pKa+log[A][HA]
A lactic acid/lactate ion buffer solution contains 0.30 M HC3H5O3 and 0.90 M C3H5O3, respectively. The Ka value of lactic acid is 1.4×104. Calculate the pH of this buffer.
Express the pH numerically.
Explanation / Answer
Ka= 1.4x10^-4
-log[Ka] = -log[1.4x10^-4]
PKa = 3.85
[HC3H5O3 ]= 0.30M
[C3H5O3-] = 0.90M
PH= PKa + log[salt]/[acid]
PH= 3.85 + log[0.90/0.30]
PH= 3.85 + 0.477
PH = 4.327
PH= 4.33.
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