If 25.0 Liters of potassium chlorate (KClO3) are used, how many grams of oxygen (O2) will form?

Answer 1

It seems like there is some missing information, possibly the number of moles or molarity of the solution.

You need the balanced equation and the number of moles of the reactant KClO₃ to determine the number of moles of O₂ produced. Then, use that number of moles times the molar weight of O₂ to determine the number of grams needed.