Acid Base Dissociation Constants (K_a and K_b) What is an acid dissociation cons
ID: 1007478 • Letter: A
Question
Acid Base Dissociation Constants (K_a and K_b) What is an acid dissociation constant, K_a? Why isn't a K_a listed for HCl? What is a base dissociation constant, K_b? Why isn't a K_b listed for NaOH? Write the chemical equation, the equilibrium constant expression, and the value of the equilibrium constant (at 25.0 degree) for the ionization for both hydrocyanic acid, and methyl amine, CH_3NH_2 Relative Acid Strengths What is the [H^+] in these two solutions: 0.250 M nitrous acid & 0.250 M nitric acid? Why are the two values that you calculated in part (a) different? Relative Acid Strengths What is [H^+] in these two solutions: 0.250 M formic acid & 0.250 M hypochlorous acid? Why are the two values that you calculated in part (a) different?Explanation / Answer
Solution:- (1) (a) ka is known as acid dissociation constant and it tells us how strong or weak the acid is. Greater value of Ka means stronger acid and least value of Ka means weaker acid.
(b) Ka for HCl is not listed because HCl is a very strong acid and for stronger acids we take complete dissociation of acid into its ions.
(c) like Ka, Kb is the base dissociation constant and it tells us how strong or weak the base is. Greater is the value of Kb means stronger the base is and least value of Kb means weaker base.
(d) Kb for NaOH is not listed since it is a stronger base and for stronger bases we take complete dissociation of the base into their ions.
(2) Hydrocyanic acid is HCN and its a weaker acid. the equation for this could be written as...
HCN + H2O <------------> H3O+ + CN-
Ka = [products]/[reactants]
we don't include H2O in this since it's taken in excess and no change to its concentration is considered. So,....
Ka = [H3O+][CN-]/[HCN]
Ka and Kb values are given in tables and the Ka value for HCN is 6.2 x 10-10.
For methyl amine, CH3NH2 the equation is.......
CH3NH2 + H2O <----------> CH3NH3+ + OH-
Kb = [CH3NH3+] [OH-]/[CH3NH2]
Kb for CH3NH2 is 4.4 x 10-4.
(3) (a) Nitrous acid is HNO2 and its a weak acid. Ka for this acid is 7.2 x 10-4. let's write the equation for its dissociation and make the ICE table.
HNO2 <--------> H+ + NO2-
I 0.250 0 0
C -X +X +X
E 0.250 - X X X
Ka = [H+] [NO2-]/[HNO2]
on plugging in the values...
7.2 x 10-4 = [X][X]/[0.250 - X]
7.2 x 10-4 = (X)2/(0.250 - X)
on cross multiply....
X2 = 7.2 x 10-4(0.250 - X)
X2 = 1.8 x 10-4 - 7.2 x 10-4X
on rearranging the equation in the form of quadratic equation, ax2 + bx + c = 0
X2 + 7.2 x 10-4X - 1.8 x 10-4 = 0
on solving this equation for X..
X = 0.013
from ice table, [H+] = X = 0.013
Nitric acid, HNO3 is a stronger acid so complete dissociation would be taking place and hence H+ would be equal to the given acid concentration. So,...
[H+] = 0.250 M
(b) Two values are different since the acid strengths are different. Being stronger acid, H+ is greater for HNO3 and being weaker it is least for HNO2.
(4) (a) Formic acid, HCOOH, is also a weaker acid and Ka for this acid is 1.8 x 10-4. So, let's make the ice table for this and calculate the value of X. where X is the change in concentration.
HCOOH <----------> H+ + HCOO-
I 0.250 0 0
C -X +X +X
E 0.250 - X X X
1.8 x 10-4 = (X)2/(0.250 - X)
on cross multiply......
X2 = 4.5 x 10-5 - 1.8 x 10-4X
X2 + 1.8 x 10-4 - 4.5 x 10-5 = 0
on solving this for X..
X = 0.00662
so, [H+] = 0.00662 M
For hypochlorous acid, HOCl the Ka is 3.5 x 10-5. Let's make the table for this and calculate the X.
HOCl <------------> H+ + OCl-
I 0.250 0 0
C -X +X +X
E 0.250 - X X X
3.5 x 10-5 = (X)2/(0.250 - X)
on cross multiply.....
X2 = 8.75 x 10-6 - 3.5 x 10-5X
X2 + 3.5 x 10-5X - 8.75 x 10-6 = 0
on solving this for X..
X = 0.00294
so, [H+] = 0.00294 M
(b) two values of H+ are different for these acids since their Ka values are different. Ka for Formic is greater than Ka of Hypochlorous acid. So, the [H+] is also greater for Formic acid as its dissociation is more.
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