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posted by  ss83 on 7/14/2008 5:30:50 PM  |  status: Live  

Print ViewPercent Dissociation

Course Textbook Chapter Problem
General Chemistry N/A 15 N/A
Question Details:
In addition to the acid-dissociation constant, K_a, another measure of the strength of an acid is percent dissociation, determined by the following formula:

\rm Percent~dissociation=\frac{[HA]~dissociated}{[HA]~initial}\times 100\%

Percent dissociation increases with increasing K_a. Strong acids, for which K_a is very large, dissociate completely
(100\%). For weak acids, the percent dissociation changes with concentration. The more diluted the acid is, the greater percent dissociation.


A certain weak acid, \rm HA, has a K_a value of 7.9×10−7.
Part A
Calculate the percent dissociation of \rm HA in a 0.10 M solution.
Express your answer as a percent using two significant figures.
 ?? \%

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Sage
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posted by Jesley on 7/23/2008 7:08:25 AM  |  status: Live
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ss83's comment:
"thanks!"
Response Details:
for the dissocia tion of acid, we write the equation as
                  HA  +  H2O -----> H3O+ +  A-
initial:        0.1M                          0            0
change:         -x                            +x         +x
equilibrium: (0.1-x)                         x           x
Ka = [H3O+ ][A-]/[HA] = 7.9x10-7
(x*x)/(0.1-x) =  7.9x10-7
on solving we get x= 0.00028M
[HA]dissociated = 0.1- 0.00028 = 0.09972M
%dissociation = [0.09972M]/[0.10M]*100 = 99.72%
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