weak acid concentration

Ethanoic acid (CH3COOH) is a weak acid. In dilute ethanoic acid, CH3COOH(aq) is partly (about 5%) ionized in water to give CH3COO^-(aq) and H^+(aq) ions.
CH3COOH(aq) = CH3COO^-(aq) + H^+(aq) ...... (*)

Water can be ionized to give H^+ and OH^- ions to an extremely small extent.
H2O(l) = H^+(aq) + OH^-(aq) ...... (#)

Since the ionization of water in reaction (#) is to an extremely small extent, but this is the only source of OH^‑ ions, then the OH^- has the smallest concentration than those in other three spcies.

CH3COOH(aq) <=> CH3COO-(aq) + H+(aq) (<=> means it is a reversible reaction)
Remember that there is a sign of reversible reaction which implies the reaction does not completely ionize and some of the formed ions will combine back to the ethanoic acid molecule.

you should know that CH3COOH is acid ,it should ionize to form H+ and CH3COO- in the water,also it is a weak acid, many of them does not ionize
so B is the least

only alkali waill form OH-
acid form H+

2010-03-09 16:42:29 補充：
common acid in exam
H3PO4 (phoroporic acid)
HCL (hydrochloric acid)
H2SO4 (sulphuric acid)
CH3COOH (ethanoic acid)

2010-03-09 16:44:54 補充：
so althought in the chemical CH3COOH . it seem like have OH- in the last two alphabets,but acid will not ionize to give OH-

2010-03-09 16:46:38 補充：
acid definition:
H+ as the only positive charge ion when it dissolve in water

base defination
OH- as the only negative charge ion when it dissolve in water