Question

1The hydroxide ion concentration of an aqueous solution of 0.563 M formic acid ,

HCOOH is [OH-] = (answer) M.

2

The hydronium ion concentration of an aqueous solution of 0.563 M acetic acid (K_a = 1.80×10^-5) is

[H₃O⁺] = (Answer) M.

Answer 1

HCOOH ↔ HCOO- + H+
Ka = [HCOO-]*[H+] / [HCOOH]
You consider two thimgs:
1) [HCOO-] = [H+]
and because [H+] is very small compared to [HCOOH], you can ignore the drop in concentration of HCOOH due to the dissociation. This applies if the dissociation is less than 5%. See below.
You can therefore write the equationas:
1.8*10^-4 = [H+]²/0.563
[H+]² = 1.01*10^-4
[H+] = 1*10^-2

pH = -log H+
pH = -log 1*10^-2
pH = 2
pOH = 14.00-2
pOH = 12
[OH-] = 10^-pOH
[OH-] = 10^-12
[OH-] = 10^-12M