[Gravemind]

I'm having a little trouble understanding Acid Base Equilibrium, its probably from the fact that I didn't fully understand regular Equilibrium.  Ive asked my teacher and some fellow students to try to help me understand but the way they explain it just doesn't work for me.  Heres an example of the stuff I'm just not understanding:

Calculate the pH change produced by adding .1moles of solid NaOH to 500 ml of 1.00M HC₂H₃O₂ and 1.00M NaC₂H₃O₂

HC₂H₃O₂->H⁺ + C₂H₃O₂^-

So we have 1mol/.5L=2
Ka = 1.8x10^-5

1.8x10^-5=[H⁺][C₂H₃O₂^-]/[HC₂H₃O₂]

1.8x10^-5=x(2+.1)/(2-.1)
x= 1.62x10^-5 which makes the pH 4.78

Now i just flip the signs for the NaC₂H₃O₂ so its:
1.8x10^-5=x(2-.1)/(2+.1)
x=1.98x10-5 which makes the pH 4.70

But when I looked at the answer it says I should've got a pH of 4.75 and 4.92 to give me a difference of .18, but I only got a difference of .08

[enahs]

You have a weak acid and it's conjugate base.
What does that mean?
What is it's initial pH?

You then have a strong base reaction with a weak acid; what happens?


Also note, always think about what is happening; do not get so focused on the numbers you miss the science. You are saying if you add a base the pH is going to go down? Does that make sense?