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 butyric acid/butyrate buffer is 0.031 M in butyric acid and has a pH of 5.11, what is the molar concentration of the butyrate ion? The pKa of butyric add is 4.81.

Explanation / Answer

Henderson-Hasselbalch equation:

pH = pKa + log([CH3CH2CH2COO-]/[CH3CH2CH2COOH])

5.11 = 4.81 + log([CH3CH2CH2COO-]/0.031)

log([CH3CH2CH2COO-]/0.031) = 0.3

[CH3CH2CH2COO-]/0.031 = 10^0.3 = 1.995

[CH3CH2CH2COO-] = 0.0619 M = 0.062 M

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