Wednesday, May 4, 2011

Adding a Strong Base to a Weak Acid

Problem: Calculate the pH of a solution when 45.0mL of 0.100M NaOH (a strong base) is added to 50.0mL of 0.100M CH3COOH (a weak acid).

HINTS:
We found that the number of mols of NaOH is 4.50 x 10^-3 and the number of mols of CH3COOH is 5.00 x 10^-3.

Since each mol of NaOH causes the weak acid to dissociate, the number of mols of CH3COO- is equal to 4.50 x 10^-3 and the remaining number of mols of CH3COOH is 5.00 x 10^-4.

We can find the Molarity of CH3COO- and CH3COOH by dividing the moles by the total volume, 0.095L.

We discussed in class that as the concentration of the conjugate base, CH3COO-, increases, it will react with water and release OH- ions.  This results in the solution becoming more basic.  Please note that the Ka accounts for this!!  So, we simply use the equation for Ka to solve for [H+].