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Relevant equations: Charge balance:
Mass balance:
Rearranging the charge balance:
Substituting into the mass balance: and
.
. Substituting into the HendersonHasselbalch equation yields: Since you know the pH and therefore
, you can simply substitute into this expression. You could
also make the familiar approximation that When you substitute in the values, you will get a pH that agrees, within experimental uncertainty, with
the measured value
.
Then you add the strong base which will shift the pH:
# moles
HA
+
OH(mol)
Initial
Final
0 A + H2O Where , the equilibrium concentrations are given in part (a,
.
Plugging into the same HendersonHasselbalch equation: and .
Where primed quantities are the solution properties after the addition of hydroxide. The approximation
we have made here (see #4, above, for derivation) is that That is, that . Checking the approximation:
, so the difference is less than
where . [BONUS – 10 marks] Consider the diprotic acid H2A with
and
.
2Using the systematic treatment of equilibrium, find the pH and concentrations of H2A, HA and A in a
0.100M aqueous solution prepared by adding NaHA to water. Justify and check any approximations that
you may make in arriving at the answer. You must derive your expressions to receive full credit.
See notes for how to derive Then use and to solve for H2A and A2, respectively....
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This document was uploaded on 03/12/2014 for the course CHM 212 at Rhode Island.
 Fall '11
 N/A

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