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Calculate the acid dissociation constant of a weak monoprotic acid if a 0.5M solution of this acid gives a hydrogen-ion concentration of 0.000 1M? Show your work.

 Apr 17, 2018
 #1
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Here we go:

 

Which undergoes HA<=> H+ + A- 
ka = [H][A] / [HA] 
you know [H+], but what are the others? 
Write the electroneutrality equation for the entire reaction (including water's dissociation) 
[H+] = [OH-] + [A-] ....we know that [H]>>>[OH], so 
[H+] = [A-] 
next write the total concentration equation (mass balance equation) 
Ct = 0.5M = [A-] + [HA] 
[HA] = 0.5 - [A-] = 0.5 - [H+] 

all together you have: 
ka = [H]^2 / (0.5 - [H+]) 
ka = 0.0001*0.0001/(0.5-0.0001) = 2.00x10^-8 

Hope you can understand this approach

 Apr 17, 2018

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