Aqueous Solutions and the Concept of pH Section 15.1

Self-Ionization of Water Self-ionization of water: two water molecules produce a hydronium ion and a hydroxide ion by transfer of a proton

Self-Ionization of Water Conductivity measurements show that the concentrations of H 3 O + and OH - in pure water are each only 1.0 x mol/L at 25° C. Concentrations are denoted by brackets

More [H 3 O + ] means “hydronium ion concentration in mol/L” In water at 25° C, [H 3 O + ] = 1.0 x M and [OH - ] = 1.0 x M The mathematical product of [H 3 O + ] and [OH - ] remains constant in water and dilute aqueous solutions at constant temperature

Ionization Constant of Water The constant mathematical product is called the ionization constant of water Denoted by K w K w (at 25° C) = [H 3 O + ] [OH - ] K w = (1.0 x )(1.0 x ) = 1.0 x the ionization of water increases as temperature increases, so K w increases

Neutral, Acidic, and Basic Solutions When [H 3 O + ] = [OH - ], the solution is neutral When [H 3 O + ] > [OH - ], the solution is acidic When [H 3 O + ] < [OH - ], the solution is basic Math alert: a smaller negative exponent means a greater concentration

Calculations Strong acids and strong bases completely dissociate so the concentration of the base equals [OH - ] and the concentration of the acid equals [H + ], also known as [H 3 O + ] What is the [H 3 O + ] of a 7.5 x M HCl solution? [HCl] = [H 3 O + ] because HCl is a strong acid [H 3 O + ] = 7.5 x M

More Practice What is the [OH - ] of the solution on the previous slide at 25°C? K w = [OH - ] [H 3 O + ] [OH - ] = K w / [H 3 O + ] [OH - ] = 1.0 x / 7.5 x [OH - ] = 1.3 x M

More Practice A 1.5 x M NaOH solution is prepared at 25°C. Calculate the [OH - ] and [H 3 O + ]. Determine if the solution is basic, acidic, or neutral and justify your answer. [OH - ] = 1.5 x M [H 3 O + ] = 6.7 x M [OH - ] > [H 3 O + ] so the solution is basic More practice: 1-3 on page 502

The pH Scale pH stands for two French words “pouvoir hydrogène” meaning “hydrogen power” pH by definition: the negative of the common logarithm of the hydronium ion concentration, [H 3 O + ]

Calculations pH = -log [H 3 O + ] A neutral solution at 25 ° C, pH = -log 1.0 x10 -7 = 7.0 [H 3 O + ] = 3.4 x 10 -4, then pH = -log 3.4 x = 3.47 So if you know the hydronium ion concentration, you can easily calculate the pH

pOH pOH: the negative of the common logarithm of the hydroxide ion concentration [OH - ] “p” means take the negative log pOH = -log [OH - ]

Relationships at 25° pH + pOH = 14.0 pH + pOH = pK w pH of an acidic solution is less than 7, while the pOH is greater than 7 pH of a basic solution is greater than 7, while the pOH is less than 7

Practice What is the pH of a solution of the [H 3 O + ] is 3.4 x M? pH = -log [H 3 O + ] pH = -log 3.4 x pH = 4.47 (always to two places past the decimal) More practice: 1-4 on page 506 (page 488 in your book at home)

Summary

Calculating Concentrations from pH and pOH [H 3 O + ] = 10 -pH What is the [H 3 O + ] of a solution with a pH of 2.34? (4.57 x M) [OH - ] = 10 -pOH What is the [OH - ] of a solution with a pOH of 12.56? (2.75 x M) What is the [OH - ] of a solution with a pH of 6.54? (3.47 x M)

Calculations Involving pH Addition practice: 1-4 on page 508 (page 490 in your book at home)

Buffer Buffered solution: a solution that can resist changes in pH when an acid or a base is added to it; a buffer If a small amount of a strong acid is added, the extra H + ions cause a shift to the left in the equilibrium and the pH is changed very little

If a small amount of a strong base is added, the extra OH - reacts with water, causes a shift to the right in the equilibrium, and the pH is changed very little

Preparation of a Buffer Solution of a weak acid and a salt of the weak acid Solution of a weak base and a salt of the weak base Add a little strong base to an excess of weak acid Add a little strong acid to an excess of weak base

Buffer Action in the Blood Human blood needs to maintain a pH of Enzymes only function over a particular pH range Because of the following buffer, blood's pH is fairly steady: CO 2(aq) + H 2 O (l) H + (aq) + HCO 3 - (aq)