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Using tabulated Ka and Kb values, calculate the pH of the following solutions. 

A solution made by mixing 100.00 mL of 1.000 M HOCl with 65.00 mL of 0.900 M NaOH.

For HOCl, Ka = 3.5×10-8.
Feb 22nd, 2015

Anytime that you have a weak acid (HOCl) and a strong base (NaOH) mixed together, AND the mols of acid > mols of base added, then you can safely use the Henderson-Hasselbalch equation.

  Henderson-Hasselbalch equation:

  pH = pka + log(A-/HA)

where   A- is the conjugate base (ion after dissociation)

  HA is the unassociated acid

First, neutralization occurs:

HA + OH- à H20 + A-

Mols of OH- added  =  0.065L * 0.900M = 5.85E-2 mols

Mols of HA before mixing =  0.10000L *  1.000M = 0.1000 mols

 

After neutralization occurs:

Mols of HA after mixing  =  0.1000mols -  5.85E-2 mols = 4.15E-2 mols

Mols of A- after mixing  = mols of OH- added =  5.85E-2 mols

Plug and chug into the Henderson Hasselbalch eqn

pH = -log(3.5E-8) + log(5.85E-2/4.15E-2)

 pH= 7.61   


Feb 22nd, 2015

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