Ka = Keq [H 2 O] = [H [CHO ]^ [CHCOCOH] ]
3 2
3 3 2 −
2) At 25 °C, the acidity constant for acetic aicd is 1.76 × 10−^5.
3) General expression for any acid:
HA + H 2 O H 3 O+ + A−
Ka = [H^3 O[HA]]^ [A ]
−
4) A large value of Ka means the acid is a strong acid, and a smaller value of Ka
means the acid is a weak acid.
5) If the Ka is greater than 10 , the acid will be completely dissociated in water.
3.5B ACIDITY AND pKa
- pKa: pKa = −log Ka
- pH: pH = −log [H 3 O+]
- The pKa for acetic acid is 4.75:
pKa = −log (1.76 × 10−^5 ) = −(−4.75) = 4.75 - The larger the value of the pKa, the weaker is the acid.
CH 3 CO 2 H CF 3 CO 2 H HCl
pKa = 4.75 pKa = 0.18 pKa = − 7
Acidity increases
1) For dilute aqueous solution: water concentration is essentially constant (~ 55.5
M)