P16B.9* (a) A dilute solution of a weak electrolyte AB, which dissociates to A* +B, is prepared with an initial concentration c. Suppose that a fraction a of AB dissociates. Assuming that activities can be approximated by concentrations, show that the equilibrium constant K for dissociation may be written a'cA K=- AB (1-a)c (b) The conductivity of the solution described in (a) is measured as K, and the molar conductivity is then calculated as A = x/CAg. However, because the degree of dissociation, and hence the concentration of the ions, varies strongly with the initial concentration cAB» values of A calculated in this way also vary strongly with cA. Given that K can be expected to be proportional to the concentration of the ions, explain why a = AIAm where Am, is the molar conductivity in the limit of complete dissociation of AB. (c) Substitute this expression for a into the above expression for K. You now have two expressions for K: one in terms of a and one in terms of A/Am1- Equate these two expressions and hence show (by rearranging your expression) that *m,l 1 A_(1-a) 1 !! m 'm,l

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P16B.9* (a) A dilute solution of a weak electrolyte AB, which dissociates to
A* +B, is prepared with an initial concentration c. Suppose that a fraction a of
AB dissociates. Assuming that activities can be approximated by concentrations,
show that the equilibrium constant K for dissociation may be written
a'cA
K=-
AB
(1-a)c
(b) The conductivity of the solution described in (a) is measured as K, and
the molar conductivity is then calculated as A = x/CAg. However, because
the degree of dissociation, and hence the concentration of the ions, varies
strongly with the initial concentration cAB» values of A calculated in this way
also vary strongly with cA. Given that K can be expected to be proportional
to the concentration of the ions, explain why a = AIAm where Am, is the
molar conductivity in the limit of complete dissociation of AB. (c) Substitute
this expression for a into the above expression for K. You now have two
expressions for K: one in terms of a and one in terms of A/Am1- Equate these
two expressions and hence show (by rearranging your expression) that
*m,l
1
A_(1-a)
1
!!
m
'm,l
Transcribed Image Text:P16B.9* (a) A dilute solution of a weak electrolyte AB, which dissociates to A* +B, is prepared with an initial concentration c. Suppose that a fraction a of AB dissociates. Assuming that activities can be approximated by concentrations, show that the equilibrium constant K for dissociation may be written a'cA K=- AB (1-a)c (b) The conductivity of the solution described in (a) is measured as K, and the molar conductivity is then calculated as A = x/CAg. However, because the degree of dissociation, and hence the concentration of the ions, varies strongly with the initial concentration cAB» values of A calculated in this way also vary strongly with cA. Given that K can be expected to be proportional to the concentration of the ions, explain why a = AIAm where Am, is the molar conductivity in the limit of complete dissociation of AB. (c) Substitute this expression for a into the above expression for K. You now have two expressions for K: one in terms of a and one in terms of A/Am1- Equate these two expressions and hence show (by rearranging your expression) that *m,l 1 A_(1-a) 1 !! m 'm,l
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