How many grams of oxygen will be obtained by decomposing 4.5 grams of water?

Given:
m (H2O) = 4.5 g

To find:
m (O2) -?

Solution:
1) Write the equation of a chemical reaction:
2H2O => 2H2 ↑ + O2 ↑;
2) Calculate the molar mass of H2O and O2:
M (H2O) = Mr (H2O) = Ar (H) * N (H) + Ar (O) * N (O) = 1 * 2 + 16 * 1 = 18 g / mol;
M (O2) = Mr (O2) = Ar (O) * N (O) = 16 * 2 = 32 g / mol;
3) Calculate the amount of substance H2O:
n (H2O) = m (H2O) / M (H2O) = 4.5 / 18 = 0.25 mol;
4) Determine the amount of substance O2:
n (O2) = n (H2O) / 2 = 0.25 / 2 = 0.125 mol;
5) Calculate the mass of O2:
m (O2) = n (O2) * M (O2) = 0.125 * 32 = 4 g.

Answer: The mass of O2 is 4 g.