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I found this question in one of the problem sets and I decided to try it, I keep getting 9.1 × 10−8 as the answer and I thought this since this is a diprotic acid and I made two equations and I keep getting that number.

Ka1: 1 × 10−7

Ka2: 9.1 × 10−8

Anyone know why? Here are the answer choices:

1.3 × 10−18

1.3 × 10−8

1.3 × 10−13

1.3 × 10−15

Why is my answer so different? For the first equation I get x = 1 × 10−4 and then for the second equation I get 9.1 × 10−8 all the time.

Please help.

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  • 1
    $\begingroup$ What are the relevant pKas? $\endgroup$ – jerepierre Feb 10 '15 at 22:44
  • $\begingroup$ @jerepierre Gave the Ka1 and Ka2 $\endgroup$ – Asker123 Feb 10 '15 at 22:45
  • $\begingroup$ What equation are you using to get x = 1E-4 and what is x? $\endgroup$ – jerepierre Feb 10 '15 at 22:47
  • $\begingroup$ I am using the Ka ionization constant to get the molarities. $\endgroup$ – Asker123 Feb 10 '15 at 22:47
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    $\begingroup$ The second Ka must be wrong. 9e-8 is pretty much the same as 1e-7, and it doesn't make sense that the second deprotonation is nearly the same as the first. Wikipedia has the second pKa as >14. $\endgroup$ – jerepierre Feb 10 '15 at 23:17
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Your value for Ka2 is incorrect.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch17/diprotic.php

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  • 1
    $\begingroup$ Yeah good point the chart that I had was incorrect. Thanks for pointing that out. $\endgroup$ – Asker123 Feb 10 '15 at 23:21

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