How many moles of ncl3 contain 2.55 * 1024 chlorine atoms?

When we have Cl atoms = 2.55 x 10^24 in NCl3

so,

1 - we need first to get moles of Cl = atoms / Avogadro's number

= 2.55x10^24 / 6.022 x 10^23

= 4.23 moles

2 - and when NCl3 contains 3 Cl atoms:

so moles of NCl3 = moles Cl / 3

= 4.23 / 3

= 1.41 moles