Advices For Studying Organic Chemistry

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

1. pKa: pKa = −log Ka


2. pH: pH = −log [H 3 O+]


3. The pKa for acetic acid is 4.75:
   pKa = −log (1.76 × 10−^5 ) = −(−4.75) = 4.75


4. 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)