# I have my acids and bases test tomorrow, and i need help with a few types of problems

im looking at some review problems my teacher gave me today, and i can pretty much do most of them. however, there is this one i really have no idea what to do. Its a 2 part problem.

1a.What is the pH of a 1000 ml of a 0.1 M acetic acid solution, given pKa = 4.8?
1b. what is the pH of 1000ml of 0.1 M acetic acid solution after you have added 5 ml of 10.0 M NaOH, assuming net volume change is negligible

so, for the first part, i think i got it wrong. But here is what i did.
First i found the Ka with
pKa = 4.8 = -log[Ka]
10^-4.8 = Ka
Ka = 1.58 x 10^5

so then i wrote out the equilibrium reaction.
C2H4O2 + H20 > C2H3O2 + H30
where everything is aqueous except H20, and the products on the right side are - and + ions, respectively.
so then
Ka = [x][x]/0.1 - x, where is x is the concentration of H30 and C2H3O2.
solving for x gives 1.25 * 10^-3

so, [Hplus] = 1.25 * 10^-3 M
then, i did this step
(1.25 x 10^-3 Hplus ions/L) * 1000ml * 1L/1000ml gives 1.25 x 10^-3 Hplus ions

pH = -log[Hplus] = 2.903

i really need help, im pretty sure i made some erroneous steps.

and for part b, i really didnt have any idea how to start. I just started by finding the number of mols of OHminus, but i dont think that does anything.
So, can someone help me do both parts?
thanks

symbolipoint
Homework Helper
Gold Member
Hi, Pakmingki,
You essentially have the right idea. In part A, you obtain about the same result whether a more full quadratic form is used or the simplification. In part B, the concentration of acid is essentially cut in half.

This is a BASIC program which can give the results that you want; if you want a more full explanation, write a private message:
let Ka=1.58*10^(-5)
let M=0.1
'note, let M=0.05 if want to
'cut concentration in half, as if

let x=Ka+sqr(Ka*Ka+4*M*Ka)
let x=x/2

pH=-1*log(x)/log(10)

print "concentration ";x
print "pH ";pH

END
You get a pH of about 3 for the addition of caustic soda, a pH of a bit less than 3 for just the acid before the addition of caustic soda.

symbolipoint
Homework Helper