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11-2
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Another T/F exercise. The points
made are subtle, but very important.
In part (b) of this problem and in problem 11-8 it is important to know that the
activity coefficient calculated from the Davies equation is the same for the
cation and its associated anion, but that the molality of the cation and the
molality of the anion must each be
multiplied by that activity coefficient.
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11-6
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Exercise in calculating ionization constants from data in thermodynamic
tables. Note that very small and very large equilibrium constants are seldom
known very well.
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11-8
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A quick calculation to show that the pH of a NaCl(aq) solution is
not 7.00, even if the temperature is 298 K. Neither the Na+(aq)
ion nor the Cl-(aq) undergoes hydrolysis, but they change
the ionic strength, which lowers the activity coefficients of the H+(aq)
and OH-(aq)
ions and so increases their concentrations.
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11-4
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(Not a short problem, but very important).
All parts require solving an equilibrium problem for a reaction involving
ions. Solutions containing ions are not ideal; activity coefficients for
uncharged molecules are usually assumed to be 1, but activity coefficients
for ions must be determined experimentally or estimated using an equation
like the Davies equation.
(a) Dissociation of formic acid in pure water.
(b) Dissociation of formic acid in a solution containing an electrolyte that
changes the ionic strength but that does not participate in the chemical
reaction being considered.
(c) Dissociation of formic acid in a solution that also contains the formate
ion, which (along with the accompanying Na+ ion) changes the ionic
strength, and which also participates in the chemical reaction being
considered (common ion effect). Note that this third solution is a buffer
because the concentrations of the formic acid (undissociated acid) and
formate ion (conjugate base) are similar.
Optional/ See how including the activity
coefficient for the uncharged, undissociated acid affects the
calculations. See problem 11.36 for
the equation.
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11-9
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It is important to remember that Kw changes with
temperature; the only temperature at which the value is 1.0 x 10-14
is 298 K.
(a) Use an empirical expression to get Kw at normal body
temperature.
(b) Estimate Kw at normal body temperature by using the
expression for the temperature dependence of ln K.
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