First part of the question: We want to calculate the concentrations of all speci
ID: 487457 • Letter: F
Question
First part of the question: We want to calculate the concentrations of all species in a 0.56 M Na2SO3 (sodium sulfite) solution. The ionization constants for sulfurous acid are Ka1 = 1.4× 10–2 and Ka2 = 6.3× 10–8. Let's start by first calculating the concentration of the spectator ion, Na+ = ?
Second part of the question: Write the appropriate equilibria and associate the correct Kb values. Remember we will want to calculate the concentrations of all species in a 1.02 M Na2SO3 (sodium sulfite) solution. The ionization constants for sulfurous acid are Ka1 = 1.4× 10–2 and Ka2 = 6.3× 10–8.
SO32-(aq) + H2O yields HSO3-(aq) + OH-(aq)
Kb value (25 degrees) = ?
HSO3-(aq) + H2O yields H2SO3(aq) + OH-(aq)
Kb value (25 degrees) = ?
Explanation / Answer
Apply:
Na2A --> 2Na+ + A-2
so A-2 is a weak acid, so it will hydrolyze to form
A-2+ H2O <---> HA- + OH- Kb2
and then
HA- + H2O <-> H2A + OH- Kb1
Recall that
Kb1 = Kw/Ka1 = (10^-14) / (1.42*10^-2) = 7.0422*10^-13
Kb2 = Kw/Ka2 = (10^-14) / (6.3*10^-8) = 1.5873*10^-7
then...
from Kb2:
Kb2 = [HA-][OH-]/[A-2]
assume, initially:
[Na+] = 2*[Na2SO3] = 2*1.02 = 2.04 M
[A-2] = 1.02
[HA-] = [OH-] = 0
in equilibrium:
[A-2] = 1.02 - x
[HA-] = [OH-] = 0 + x
substitute and solve:
7.0422*10^-13 = (x)(x)/ (1.02-x)
solving for x:
x =8.47*10^-7
so...
[A-2] = 0.1 - x= 1.02-8.47*10^-7 = 1.0199
[HA-] = [OH-] = 0 + 8.47*10^-7 = 8.47*10^-7
now...
next equilibrium
Kb1 = [H2A][OH-]/[HA-]
assume
[H2A] = 0
[OH-] = x = 8.47*10^-7
[HA-] = x= 8.47*10^-7
in equilibrium:
[H2A] = 0 + y
[OH-] = x + y = 8.47*10^-7 + y
[HA-] = x - y= 8.47*10^-7 - y
so... substitute
Kb1 = [H2A][OH-]/[HA-]
1.5873*10^-7 = (y)(8.47*10^-7 + y) / ( 8.47*10^-7 - y)
solve for y:
(1.5873*10^-7)(8.47*10^-7) - (8.47*10^-7)(y) = (y)(8.47*10^-7) + y^2
y^2 + (0.000001694)y - 1.344*10^-13 = 0
y = 1.339*10^-13
so:
[H2A] = 0 + y = 1.339*10^-13
[OH-] = x + y = 8.47*10^-7+ +1.339*10^-13 = 8.47*10^-7
[HA-] = x - y= 8.47*10^-7+ -1.339*10^-13 = 8.340*10^-7
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.