Calculate the volume of O2 that is formed during the decomposition of 140 g of potassium nitrate?

The reaction equation for the decomposition of potassium nitrate:

2KNO3 (t) = 2KNO2 + O2

Amount of potassium nitrate substance:

v (KNO3) = m (KNO3) / M (KNO3) = 140/101 (mol).

According to the reaction equation, from 2 mol of KNO3, 1 mol of O2 is formed, therefore:

v (O2) = v (KNO3) / 2 = (140/101) / 2 = 70/101 (mol).

Thus, the volume of oxygen evolved under normal conditions (n.o.):

V (O2) = v (O2) * Vm = (70/101) * 22.4 = 15.5 (l).