Since it talks about the number of particles, we would need the Avogadro's number which is an empirical value equal to 6.022×10²³ particles/mole. Therefore, to determine the number of particles, such as molecules, just multiple the number of moles to the Avogadro's number:
Number of Br₂ molecules = 0.42 mol *6.022×10²³ molecules/mole = 2.53×10²³ molecules
