How many atoms are present in 4.0 mol of sodium

1 mole = 6.02 x 10^23 atoms (of any element)

So we are going to take our known value of 4 moles, and multiply by 6.02 x 10^23 (Avogadro's number) and we will get the number of atoms that are in 4 moles. 

(4.0 moles of Na) x (6.02 x 10^23) / (1 mole) = 2.4 x 10^24 atoms of Na
There are 2.4 x 10^24 atoms of Na in 4.0 moles. 

A.

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.