Chapter 15 Acid-Base Equilibra (Student)

Chapter 15 Acid-Base Equilibra (Student) - 15.0 Acid-Base...

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15.0 Acid-Base Equilibria 15.1 pH of the Acid and Base Solution a) Strong Acid and Strong Base A strong acid and a strong base is an acid or a base that is essentially 100 % dissociated in aqueous solution to produce a high concentration of H 3 O + or OH - . The degree of dissociation, α = 1 or 100% In strong acids and bases, the H 3 O + or OH - concentration can be obtained from the original concentration of the acid or base used. For example: HCl → H + + Cl - 0.5 M 0.5M NaOH → Na + + OH - 0.1 M 0.1M H 2 SO 4 → 2H + + SO 4 2 - ( assume that the acid is 100% 0.2M 0.4M dissociated) Example 15.1: Calculate the pH of the following solutions: 1. An aqueous solution contains 0.700 g NaOH in 485 mL water. NaOH is a strong base: NaOH → Na + + OH - 1 mole NaOH No of moles = 0.700 g NaOH x 40 g NaOH = 0.0175 moles of NaOH 0.0175 moles of NaOH 485 x 10 -3 L = 0.0361 M
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pOH = - log [ OH - ] = - log 0.0361 = 1.442 pH = 14 - pOH = 14 - 1.442 = 12.558 2. A solution of 5 x 10 -3 M H 2 SO 4 H 2 SO 4 → 2H + + SO 4 2 - 5 x 10 -3 2(5 x 10 -3 ) pH = - log [ H + ] = - log (1 x 10 -2 ) = 2 Exercise 15.1.2 1. Calculate the H + and OH - concentration in solutions with pH: a) 2.50 b) 8.75 c) 11.73 2. What is the NaOH concentration in a solution with pH = 10.50. 3. How many grams of NaOH is needed to prepare 250 mL solution with pH = 12.40? b) Weak Acid and Weak Base
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Very few acids and bases can donate or accept protons, respectively. The vast majority of acids and bases are weak. Weak acid and weak base only partially ionize in an aqueous solution. At equilibrium, an aqueous solution of weak acid or weak base contains a mixture of non-ionized acid or base molecule and its conjugate base or acid, respectively. One way to specify the extent of the dissociation of weak acid or base is by giving its percent dissociation which is defined as, mole/L acid or base dissociated % dissociation = mole/L acid or base available For example, the amount of H + dissociated in a solution of 0.10 M acetic acid is 1.3 x 10 -3 M, the percent dissociation is: 1.3 x 10 -3 = x 100 0.1 = 1.3 % Consider a general weak acid, HA. The ionization equation in water is given by: HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Given, c = initial concentration x = concentration of H 3 O + ion dissociated HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) Initial concentration Change Equilibrium concentration c - x c - x 0 +x x 0 +x x The ionization expression is given by: [ H 3 O + ] [ A - ] K a = [ HA ] where the subscript ‘a’ indicate that it is an equilibrium constant for a weak acid in water. K a is called an acid-dissociation constant . (x)(x)
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K a = (c - x) Assuming, that only a small amount of acid dissociates, i.e. K a <<< 1, therefore: [ HA ] = c - x ~ c x 2 K a = c x 2 = K a .c x = K a . c = [ H 3 O + ] Example 15.2: Calculate the pH of 0.10 M acetic acid solution , given K a for acetic acid is 1.8 x 10 -5 M. The dissociation of acetic acid in water is given by:
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Chapter 15 Acid-Base Equilibra (Student) - 15.0 Acid-Base...

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