A buffer consists of a weak acid or base, and the salt of that acid or base. Con
ID: 1017818 • 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 0.48 M HC3H5O3 and 0.74 M C3H5O3, respectively. The Ka value of lactic acid is 1.4×104. Calculate the pH of this buffer.
Explanation / Answer
A:- pH = -log Ka + log [C3H5O3] / [HC3H5O3]
= -log (1.4 * 10^-4) + log (0.74) / (0.48)
= 3.854 + log (1.542)
= 3.854 + 0.1881
pH = 4.04
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