HClO is a weak acid (Ka 4 010 8) and so the salt NaClO acts asa weak b...

HClO is a weak acid (Ka=4.0Ã—10^âˆ’8) and so the salt NaClO acts as a weak base....

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Chemistry

HClO is a weak acid (Ka=4.0Ã—10^âˆ’8) and so the salt NaClO acts asa weak base. What is the pH of a solution that is 0.037 M in NaClOat 25 Â°C?

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3.7 Ratings (532 Votes)

use:
Kb = Kw/Ka
Kw is dissociation constant of water whose value is 1.0*10^-14 at 25 oC
Kb = (1.0*10^-14)/Ka
Kb = (1.0*10^-14)/4*10^-8
Kb = 2.5*10^-7
ClO- dissociates as

ClO-Â Â Â Â Â Â Â + H2OÂ Â ----->Â Â Â Â HClO +Â Â OH-
0.037Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â 0Â Â Â Â Â Â Â Â 0
0.037-xÂ Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â xÂ Â Â Â Â Â Â Â x

Kb = [HClO][OH-]/[ClO-]
Kb = x*x/(c-x)
Assuming x can be ignored as compared to c
So, above expression becomes

Kb = x*x/(c)
so, x = sqrt (Kb*c)
x = sqrt ((2.5*10^-7)*3.7*10^-2) = 9.618*10^-5

since c is much greater than x, our assumption is correct
so, x = 9.618*10^-5 M

use:
pOH = -log [OH-]
= -log (9.618*10^-5)
= 4.0169

use:
PH = 14 - pOH
= 14 - 4.0169
= 9.9831
Answer: 9.98

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