[Boxxxed]

c.  Calculate the [CH3COOH], [CH3COO-] and the pH of a 0.1 M acetic acid solution at equilibrium. (5 points)



Answer:  [CH3COOH] =                       [CH3COO-] =                         pH = [/b]


How do I start this question?

I will need to derive a ratio somewhere and use the 0.1 moles as a total to calculate concentrations and then pH. Knowing that the buffer is in equilibrium what information does this give me?

Do I calculate the disassociation of the acid using its ka value?

ka= x² / 0.1-x

ka = x² / 0.1

x = Sqroot (1.8x10^-5)(0.1)
x = 0.0013432 moles disassociated

pH = 2.87

Concentration of base = 0.001342 M
Concentration of Acid  = 0.1-0.001342 = 0.098658 M

[Boxxxed]

And for the second part

What mass of sodium acetate, NaCH3COO,  must be added to 1 L of 0.1 M acetic acid solution to make a buffer with a pH = 4.8 ?




ratio for pH of 4.8 is 1.12

Would the correct approach then be

0.001342+x
------------ = 1.12
0.098658-x

x = moles of sodium acetate.

[rabolisk]

Looks right to me. The first question has nothing to do with buffers, since you don't have a buffer. All you need to do is what you did, which is use the Ka. I would report pH as 2.9, since it is technically 1 sig fig. For the second problem, you have the correct idea. You can ignore 0.001342 and 0.098658 though, and just use 0 and 0.1, respectively.