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Determine volume of HCl added to propionic acid to get pH=1

by BThomas1219
Tags: acid, added, determine, propionic, volume
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BThomas1219
#1
Aug4-11, 12:58 AM
P: 3
1. The problem statement, all variables and given/known data
Given 250ml of 0.1M HC3H5O2 (Ka=1.35E-5) What volume of
a)1.0M HCl is needed to lower the pH to 1.00.
b) 1.00M NaC3H5O2 raise pH to 4.00


2. Relevant equations
pH=pKa-log (HA/A)
ICE chart

3. The attempt at a solution
I used the ice chart for the dissociation of propionic acid. I determined the concentration of H+ and A- to be 1.16E-3. the pH was 2.93. I do not know how to factor in the HCl to determine the volume needed to lower the pH.
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Borek
#2
Aug18-11, 03:32 AM
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P: 23,363
[tex]K_a = \frac {[A^-][H^+]}{[HA]}[/tex]

rearranged:

[tex]\frac {[A^-]}{[HA]} = 10^{pH - pK_a}[/tex]

At pH 1.0 only about 101-4.75=10-3.75=2x10-4 of the acetic acid is dissociated. That means in the first case you can - more or less safely - assume presence of acetic acid doesn't matter much, as the first approximation pH will depend mostly on the HCl dilution.

Second case is just a buffer solution, use Henderson-Hasselbalch equation.


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