To understand and find the answer to the question, there has to be an understanding of what is Avogadro's Number. Avogadro's Number (6.02 × 10²³ particles/mole) represents the number of particles (molecules/atoms) that are in one mole of a substance.
Thus, if 1 mole contains 6.02 × 10²³ particles
Then let x moles contain 1.23 × 10¹⁸ particles
⇒ (6.02 × 10²³) x = 1.23 × 10¹⁸
⇒ x = (1.23 × 10¹⁸) ÷ (6.02 × 10²³)
⇒ x = 2.043 × 10⁻⁶ moles OR 0.000002043 moles
Thus the number of moles of water that are in 1.23 × 10¹⁸ water molecules are
2.043 × 10⁻⁶ moles OR 0.000002043.
