Carboxylic acids are weak acids and dissociate in aqueous solution to the carbox

Question

Carboxylic acids are weak acids and dissociate in aqueous solution to the carboxylate anion.

The extent of dissociation is given by the equilibrium dissociation constant, Ka, which is often expressed in its logarithmic form as pKa. (The cation is written simply as H and is understood to be solvated with one or more water molecules in solution.)

If a formic acid/formate buffer is 0.034 M in formic acid and has a pH of 3.85, what is the molar concentration of the formate ion? The pKa of formic acid is 3.75.

Explanation / Answer

We will use Hendersen Hassalbalch equation of buffers

pH = pKa + log[salt ] / [acid]

pH = 3.85

pKa = 3.75

[acid] = 0.034

[salt] = ?

3.85 = 3.75 + log [salt] / 0.034

0.10 = log [salt] / 0.034

Take antilog

1.25 = [salt] / 0.034

[salt] = 1.25 X 0.034 = 0.0425

So the molar concentration of salt os 0.0425M

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