Answer is: 5,1 mol KOH.
V (KOH) = 0,8l = 0,8 dm³.
c (KOH) = 6,4 mol/dm³.
V - volume of solution.
c - concentration of solution.
c = n÷V
n - amount of substance.
n (KOH) = c·V = 6,4 mol/dm³ · 0,8 dm³ = 5,12 mol.
Potassium hydroxide (KOH) can dissolve in water at room temperature.