Calculate the mass of potassium permanganate KMnO4, the complete decomposition of which resulted in potassium

Calculate the mass of potassium permanganate KMnO4, the complete decomposition of which resulted in potassium manganate K2MnO4, manganese dioxide MnO2 and 5.6 liters of oxygen.

Reaction equation: 2KMnO4 = K2MnO4 + MnO2 + O2
Let’s find the amount of oxygen: n = V / Vm = 5.6 l / 22.4 l / mol = 0.25 mol. (Vm – molar volume, constant for all gases under normal conditions).
According to the reaction, 1 mol of oxygen is formed from 2 mol of permanganate, n (KMnO4) / 2 = 0.25 / 1. n (KMnO4) = 2 * 0.25 = 0.5 mol. Then the mass of permanganate will be: m = n * M = 0.5 mol * (39 + 55 + 16 * 4) g / mol = 79 g (M is the molar mass, determined from the periodic table).
Answer: the mass of permanganate is 79 g