How many grams of water is formed if oxygen reacts with hydrogen in an amount of 0.5 mol.

Given:
n (O2) = 0.5 mol

To find:
m (H2O) -?

Decision:
1) Write the equation of a chemical reaction:
2H2 + O2 => 2H2O;
2) Determine the amount of substance H2O (taking into account the coefficients in the reaction equation):
n (H2O) = n (O2) * 2 = 0.5 * 2 = 1 mol;
3) Using the periodic system of D.I.Mendeleev, calculate the molar mass of H2O:
M (H2O) = Mr (H2O) = Ar (H) * N (H) + Ar (O) * N (O) = 1 * 2 + 16 * 1 = 18 g / mol;
4) Calculate the mass of H2O:
m (H2O) = n (H2O) * M (H2O) = 1 * 18 = 18 g.

Answer: The mass of H2O is 18 g.