Calculate the amount of atomic oxygen substance that is contained in a berthollet salt weighing 4.9 g.

Given:
m (KClO3) = 4.9 g

To find:
n (O in KClO3) -?

Decision:
1) Using the periodic system of D.I.Mendeleev, calculate the molar mass of KClO3, which will be numerically equal to the molecular mass:
Mr (KClO3) = Ar (K) + Ar (Cl) + Ar (O) * 3 = 39 + 35.5 + 16 * 3 = 122.5 g / mol;
2) Find the amount of substance KClO3:
n (KClO3) = m (KClO3) / Mr (KClO3) = 4.9 / 122.5 = 0.04 mol;
3) Based on the definition of “amount of substance”, find the amount of substance O in KClO3 (using proportion):
1 mol KClO3 – 3 mol О;
0.04 mol KClO3 – x mol O;
x = 0.04 * 3/1 = 0.12 mol;
n (O in KClO3) = x = 0.12 mol.

Answer: The amount of substance O in KClO3 is 0.12 mol.