The number of molecules in 54 g of water is?

Given:
m (H2O) = 54 g
NA = 6.02 * 10 ^ 23 mol − 1

Find:
N (H2O) -?

Solution:
1) 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;
2) Calculate the amount of substance H2O:
n (H2O) = m (H2O) / M (H2O) = 54/18 = 3 mol;
3) Calculate the number of H2O molecules:
N (H2O molecules) = n (H2O) * NA = 3 * 6.02 * 10 ^ 23 = 1.81 * 10 ^ 24.

Answer: The number of H2O molecules is 1.81 * 10 ^ 24.