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ACIDS AND BASES Dissociation Constants. weaker the acid, the stronger its conjugate base stronger the acid, the weaker its conjugate base.

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Presentation on theme: "ACIDS AND BASES Dissociation Constants. weaker the acid, the stronger its conjugate base stronger the acid, the weaker its conjugate base."— Presentation transcript:

1 ACIDS AND BASES Dissociation Constants

2 weaker the acid, the stronger its conjugate base stronger the acid, the weaker its conjugate base

3 stronger the base, the weaker its conjugate acid weaker the base, the stronger its conjugate acid

4  Write equilibrium expression for an acid or base  Calculate the acid/base dissociation constant  Calculate the percent dissociation

5 HA (aq) H + (aq) + A - (aq) K a - acid dissociation constant HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Strong Acid Weak Acid Larger K a : strong acid: more product : more H +.

6 BOH (aq) B + (aq) + OH - (aq) Larger K b : strong base : more product : more OH -. K b - base dissociation constant Strong Base B (aq) + H 2 O (l) BH + (aq) + OH - (aq) Weak Base

7 Initially a 0.10 M solution of acetic acid, it reached equilibrium with a [H 3 O + ] = 1.3 x 10 -3 M. What is the acid dissociation constant, K a ? CH 3 COOH (aq) + H 2 O (l) H 3 O + (aq) + C 2 H 3 O 2 ¯ (aq) I 0.10 0 0 C -1.3 x 10 -3 +1.3 x 10 -3 +1.3 x 10 -3 E 0.987 1.3 x 10 -3 1.3 x 10 -3

8 K a = 1.7 x 10 -5 There are no units for the Ka value

9 HA is a weak acid with a K a of 7.3 x 10 -8. What are the equilibrium concentrations (HA, H 3 O + and A¯) if the initial concentration of HA is 0.50 mol/L? I 0.50 0 0 C -x +x+x E 0.5-x +x +x HA (aq) + H 2 O (l) H 3 O + (aq) + A¯ (aq)

10 *K a is small - assume that x is negligible compared to 0.50

11 [H 3 O + ] = [A¯] = x = 1.9 x 10¯ 4 mol/L [HA] = 0.50 - x = 0.50 - 1.9 x 10¯ 4 = 0.49981 mol/L [HA] = 0.50 mol/L *K a is small - assume that x is negligible compared to 0.50

12 Calculate the pH of a 0.10 mol/L hydrogen sulfide solution. (K a =1.0 x 10 -7 ) H 2 S (aq) + H 2 O (l) H 3 O + (aq) + HS - (aq) I 0.10 00 C -x+x+x E 0.10 - xxx

13 [H 3 O + ] = x = 1.0 x 10 -4 mol/L pH = -log [H 3 O + ] = -log(1.0 x 10 -4 ) pH = 4.00

14 Each acid/base has K associated with it. Diprotic/triprotic acids lose their hydrogens one at a time - Each ionization reaction has separate K a. Sulfuric acid H 2 SO 4 H 2 SO 4(aq) H + (aq) + HSO 4 ¯ (aq) HSO 4 ¯ (aq) H + (aq) + SO 4 -2 (aq) K a1 K a2

15 Percent Dissociation The dissociation constants represent the acid / base degree of dissociation. Another way to describe the amount of dissociation is by percent dissociation.

16 Calculate the percent dissociation of a 0.100 M solution of formic acid (CH 2 OOH) if the hydronium ion concentration is 4.21 x 10 -3 M. CH 2 O 2 H (aq) + H 2 O (l) H 3 O + (aq) + CH 2 O 2 ¯ (aq)

17 Calculate the K b of hydrogen phosphate ion (HPO 4 2 ¯) if a 0.25 mol/L solution of hydrogen phosphate is dissociated is 0.080%. HPO 4 2 ¯ + H 2 O H 2 PO 4 ¯ + OH¯ [OH - ] = [H 2 PO 4 - ] = 2.0 x 10 -4 mol/L

18 HPO 4 2 ¯ + H 2 O H 2 PO 4 ¯ + OH¯

19  The smaller the K a or K b, the weaker the acid / base  The percent dissociation also describes the amount of acid/base dissociated  The percent dissociated is calculated by


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