Calculate the volume of O2 that will be required to react with 15 liters of H2 to form water.

Given:
V (H2) = 15 l

Find:
V (O2) -?

1) Write the equation of a chemical reaction:
O2 + 2H2 => 2H2O;
2) Calculate the volume of O2 (under normal conditions), guided by the Gay-Lussac law (the law of volumetric ratios, taking into account the coefficients in the reaction equation):
V (O2) = V (H2) / 2 = 15/2 = 7.5 liters.

Answer: The O2 volume is 7.5 liters.