How many grams of oxygen is needed to react with 1.8 g aluminum?

Given:
m (Al) = 1.8 g

To find:
m (O2) -?

Decision:
1) Write the reaction equation:
4Al + 3O2 => 2Al2O3;
2) Calculate the molar mass of Al:
M (Al) = Mr (Al) = Ar (Al) = 27 g / mol;
3) Calculate the amount of Al substance:
n (Al) = m (Al) / M (Al) = 1.8 / 27 = 0.07 mol;
4) Determine the amount of substance O2:
n (O2) = n (Al) * 3/4 = 0.07 * 3/4 = 0.05 mol;
5) Calculate the molar mass of O2:
M (O2) = Mr (O2) = Ar (O) * N (O) = 16 * 2 = 32 g / mol;
6) Calculate the mass of O2:
m (O2) = n (O2) * M (O2) = 0.05 * 32 = 1.6 g.

Answer: The mass of O2 is 1.6 g.