Calculate the amount of substance corresponding to 54 g of water.

Let us find the molar mass of water as the sum of the atomic masses of its constituent elements, multiplied by their corresponding indices:

M (H2O) = A (O) + 2 * A (H) = 16 + 2 * 1 = 18;

The amount of a substance is calculated as the ratio of the mass of a substance to its molar mass:

n (H2O) = m (H2O) / M (H2O) = 54/18 = 3 mol.

Answer: n (H2O) = 3 mol.