Calculate the mass of potassium chlorate consumed to obtain 5 liters of oxygen.

Given:
V (O2) = 5 l

To find:
m (KClO3) -?

Decision:
1) write the reaction equation (conditions: cat., T):
2 KClO3 => 2 KCl + 3 O2 ↑

2) find the amount of substance O2:
n (O2) = V (O2) / Vm = 5 / 22.4 = 0.22 mol

3) find the amount of substance KClO3 (according to the reaction equation):
n (KClO3) = n (O2) * 2/3 = 0.15 mol

4) find the mass of the substance KClO3:
m (KClO3) = n (KClO3) * Mr (KClO3) = 0.15 * 122.5 = 18.38 g

Answer: The mass of potassium chlorate is 18.38 g.