Hey there!
1 mol is 6.022 x 10²³ formula units.
To find the amount of formula units in 5.81 mol of BaCl₂, we need to multiply the amount in 1 mol by 5.81.
5.81 x 6.022 x 10²³
35.0 x 10²³
3.40 x 10²⁴
There are 3.4 x 10²⁴ formula units in 5.81 mol BaCl₂.
Hope this helps!