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April 20, 2014

Homework Help: Chemistry, acids and bases

Posted by Marissa on Tuesday, May 6, 2008 at 8:45pm.

(a) Find pH and pOH of 1.0 M solution of sodium benzoate, NaC6H5COO. (Ka of benzoic acid is 6.2x10^-5)
(b) Calculate pH after 0.205 moles per liter of HCl was added (assume volume does not change
(c) Calculate pH after 1.0 moles per liter of HCl was added (assume volume does not change)

So for part (a) I know how to start it:
We have
NaC6H5COO (aq) <---> C6H5COO- (aq) + Na+ (aq)
C6H5COO- will react with water:

C6H5COO- (aq) + H2O (l) <---> C6H5COOH (aq) + Na+ (aq)

initial M 1.0 <---> 0 + 0
change -x <---> +x , +x
equilib 1.0-x <---> x, x

Since we're solving for the base C6H5COO-

Kb = [C6H5COOH][OH-] / [C6H5COO-]

But I have the Ka, not Kb. Would I just plug Ka in for C6H5COOH?


For part (b)
NaC6H5COO- (aq) + HCl (aq) <---> NaCl (aq) + C6H5COOH (aq)

I'm left with 0.8 NaC6H5COO-, 0.205 NaCl, 0.205 C6H5COOH so I can use Henderson-Hasselbach equation

pH = pKa + log (0.8/0.205)

Am I going to use the pKa of the Ka of benzoic acid given?

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