Acids and Bases

An Acid is a substance that can release a proton or hydrogen ion (H+ )when dissolved in water” HCl H+ + Cl- . A base is a substance that accepts a proton .

A base can be defined as a substance that can release OH- ions when dissolved in water . NaOH OH- + Na+ . “ A Base is a substance that can release a Hydroxyl ion when dissolved in water”

Example of conjugate acid – base pairs CH3COOH / CH3COO- H2CO3 / HCO3-

Weak acids and aeak bases

Weak acids . Weak acids and bases dissociate weakly in solutions thus they are predominantly present in the molecular form. Dissociation of weak acids Ka; In aqueous solutions weak acids ionize to a very limited extent as follows; HA + H2O H3O+ + A- Ki = [H3O+ ] [A- ] [H2O][HA] Since [H2O] is constant a new constant ka can be introduced , ka = [H3O+ ] [A- ] [HA]

Dissociation of water Water molecules can dissociate in water as follows, H2O H + + OH- The tendency of water to ionize is expressed as Ki . Ki = [ H + ] [OH- ] [ H2O] Calculation of Ki ; Since 1 mole of H2O weighs 18g , thus the number of molecules in 1l of water (1000g) No of moles of water in 1 l = wt / MWt = 1000/ 18 = 55.56mole So the molarity of pure water is 55.5M. Since the probability that a hydrogen is present as an ion is 1.8 x10-9 thus the Molar concentration of both [ H + ] and [OH- ] can be calculated as 1.8 x10-9 x 55.56 = 1.0 x 10-7 M . Thus Ki = [ H + ] [OH- ] = 10-7 x 10-7 = 1.8 x 10-16 M . [ H2O] 55.56

Ionization of water To calculate for kw ; Ki = [ H + ] [OH- ] [ H2O] Ki [ H2O] = [ H + ] [OH- ] 1.8 x 10-16 x 55.56 = 1x10-14 = [ H + ] [OH- ] So kw = [ H + ]+ [OH- ] = 1x10-14 Taking -log of both sides -log kw = -log [ H + ] -log[OH- ] p kw = p [ H + ] + p[OH- ] = 7 + 7 = 14