1 mol of any substance is made of 6.022 x 10²³ units.
When its 1 mol of a formula, its made of 6.022 x 10²³ formula units
Therefore if 6.022 x 10²³ formula units of CeI₃ make up 1 mol of CeI₃
Then 9.42 x 10²¹ formula units make up - 1/(6.022 x 10²³) x (9.42 x 10²¹) = 0.0156 mol of CeI₃
Therefore number of moles of CeI₃ - 0.0156 mol
Subtract the number of protons from the mass number.
Before making a hypothesis, a scientist should make observations and conduct research.
I hope this helps.
