We have given:
Number of moles = Given mass/Molar mass
Given mass = 90g
Molar mass = 18g
Therefore, number of moles = 90/18 = 5
Number of moles
= Number of particles/Avogadro number
Number of particles = Number of moles × Avogadro number
Number of particles = 5 x 6.022 x 1023
= 30.11 x 1023 molecules