Determine Ka of a monohydric acid if the pH=2 and the initial concentration is 0.445 mol/L!

First of all we have to determine how much of the acid is dissociated, which we can do by determining the concentration of Hydrogen ions. We can work that out from the pH. pH=2, which means [H⁺]=10^-2 mol/L.  That means 0.01 mol/L is dissociated, beacuse t's monohydric, therefore the c of H⁺ equals to the c of dissociated acid. To figure out Ka we can use the equation:
K_a= [H⁺][anions of acid]/ [undissociated acid] 
In this example we can get the undissociated acid like this: amount of acid- dissociated acid= 0.445 mol/L-0.01mol/L=0.435 mol/L

K_a=0.01 mol/L x 0.01 mol/L / 0.435 mol/L= 2.30 x 10^-4 mol/L
We usually give the answer ina standard form and one should never forget about units.