How many grams of oxygen can be obtained by decomposition of 245 g of berthollet’s salt?

Given:
m (KClO3) = 245 g

To find:
m (O2) -?

Decision:
1) Write the reaction equation:
2KClO3 => 2KCl + 3O2 ↑;
2) Calculate the molar masses of KClO3 and O2:
M (KClO3) = Mr (KClO3) = Ar (K) * N (K) + Ar (Cl) * N (Cl) + Ar (O) * N (O) = 39 * 1 + 35.5 * 1 + 16 * 3 = 122.5 g / mol;
M (O2) = Mr (O2) = Ar (O) * N (O) = 16 * 2 = 32 g / mol;
3) Calculate the amount of KClO3 substance:
n (KClO3) = m (KClO3) / M (KClO3) = 245 / 122.5 = 2 mol;
4) Determine the amount of substance O2:
n (O2) = n (KClO3) * 3/2 = 2 * 3/2 = 3 mol;
5) Calculate the mass of O2:
m (O2) = n (O2) * M (O2) = 3 * 32 = 96 g.

Answer: The mass of O2 is 96 g.