Question

1. (Answer c)The hydroxide ion concentration in a 1.0 mol/L solution of acetic acid at 25°C is
a.4.2 x 10^-3M
b.2.4 x 10^-11M
c.2.4 x 10^12M
d.2.4 x 10^-3M
e.1.8 x 10^-5M

I did [OH-]=root(Kw/Ka) x [salt]

[OH-]=root(1.00 x 10^-14/1.8 x 10^-5) x [1]
I don't get the desired answer.

2. (Answer D)

Barium hydroxide completely dissociates in aqueous solution. What is the [OH-(aq)] in a 0.5 mol/L solution of barium hydroxide?
a. 0.5 mol/L
b. 0.25 mol/L
c. 0.75 mol/L
d. 1.0 mol/L
e. 0.1 mol/L
How do I go about solving this question?

Answer 1

.........HAc => H^+ + Ac-
I........1......0......0
C........-x.....x......x
E......1-s......x......x

Ka = 1.8E-5 = (x^2)/1
Solve for x = H^+.
Then OH = Kw/H^_. Answer c is correct.

2. Simple.
.........Ba(OH)2 ==> Ba^2+ + 2OH^
So if (OH)2 = 0.5 and it dissociates completely, then (Ba^2+) must be 0.5 and (OH^-) must be twice that. Answer D is correct.-

Answer 2

I have no idea why I answered that way other than it was way past my bed time. OK, now I see. Instead of (OH)2 it should be Ba(OH)2. It should read as follows:
So if Ba(OH)2 is 0.5M and it dissociates completely, then (Ba^2+) = 0.5 M and (OH^-) is twice that. Answer D is correct.

Regarding the first problem I see I made a typo. That last sentence should read as follows:
Then (OH^-) = Kw/(H^+). Answer c is correct.