A mole of anything is 6.022 x 10^23 particles
So 0.25 mol CO has 0.25 mol of CO molecules Β
0.25 mol CO has 0.25 x 6.022 x 10^23 molecules = 1.5 x 10^23 molecules [to 2 sig figs]
[btw ... if it wanted the number of atoms ... it'd be twice as much because there'd be 0.25 mol of C atoms + 0.25 mol of O atoms ... just be careful what they ask for]
2.00 mol Ni = 2.00 x 6.022 x 10^23 atoms = 1.20 x 10^24 atoms [to 3 sig figs]
HMMM ... looks like you use 6.02 x 10^23 for the mole
so 3.01 x 10^23 atoms He is 0.5 mol He [because He is monatomic]
Molar mass He = 4.00 g
Mass He in sample = 0.5 mol He x (4.00 g He / 1 mol He)
= 2.00 g [to 3 sig figs]
Neon is also monatomic
Molar mass Ne = 20.18 g
Number of mol Ne in sample = 80 g Ne x (1 mol Ne / 20.18 g Ne)
Number of atoms in sample = 80 g Ne x (1 mol Ne / 20.18 g Ne) x 6.022 x 10^23
= 2.4 x 10^24 atoms of Ne [to 2 sig figs]
um hope this is what u meant ^-^