How many molecules are in 3.50 moles of H2O

2.11 x 10²⁴ molecules.

Explanation:

It is known that every 1.0 mole of a molecule contains Avogadro's number of molecules (NA = 6.022 x 10²³).

Using cross multiplication:

1.0 mole of H₂O contains → 6.022 x 10²³ molecules.

3.5 mole of H₂O contains →? molecules.

∴ 3.5 mole of H₂O contain = (3.5 mol) (6.022 x 10²³) = 2.11 x 10²⁴ molecules.