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Author Topic: pH of NaHCO3 solution why Ph=8 ???  (Read 17129 times)

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jbarradas

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pH of NaHCO3 solution why Ph=8 ???
« on: July 14, 2007, 08:49:28 AM »

NaHCO3---> Na+   +    HCO3-

HCO3- H+---> H2CO3

so Ka H2CO3=4,2*10^-7

so[HCO3-]*[H+]/H2CO3 = 4,2*10^-7

thats why 0,1M NaHCO3 should make pH=12,3... but its only 8,1

I can not understand what I am missing
please anyone could help me?
« Last Edit: July 14, 2007, 01:21:49 PM by jbarradas »
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Yggdrasil

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Re: pH of NaHCO3 solution why Ph=8 ???
« Reply #1 on: July 14, 2007, 11:38:32 PM »

You also have this equilibrium:

HCO3- <--> H+ + CO32-
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Borek

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Chembuddy chemical calculators - stoichiometry, pH, concentration, buffer preparation, titrations.info, pH-meter.info

the17train

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Re: pH of NaHCO3 solution why Ph=8 ???
« Reply #3 on: July 26, 2007, 11:47:56 AM »

Where did 12.3 come from? I'm getting 10.3 when using Ka=4.3*10^-7.
Ka=[OH-][H2CO3]/[HCO3-]=     x^2/(.1)
Therefore x=[OH-]=.000207
pOH=-log(.000207)
14-3.6833=10.32
pH=10.32
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AWK

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Re: pH of NaHCO3 solution why Ph=8 ???
« Reply #4 on: July 26, 2007, 07:27:26 PM »

pH=-log(SQRT(K1xK2) = 8.34 in this case, practically pH only slightly depends on concetration
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