Exercise 2 solution

Molecules: N = n x N_A
  N = 0.250 mol x 6.02 x 10^23 /mol = 1.505 x 10^23, so 1.51 x 10^23 molecules
  (Python with the full constant gave 1.5055 x 10^23).

Atoms: each H2O has 3 atoms (2 H and 1 O), as the formula shows (Chapter 5).
  total atoms = 3 x 1.505 x 10^23 = 4.52 x 10^23

Hydrogen atoms: 2 per molecule.
  H atoms = 2 x 1.505 x 10^23 = 3.01 x 10^23

Note: the question asks about molecules because water is a molecular compound.
The numbers were calculated in Python (check, my own arithmetic).