(a)how much energy in KJ/mol is released when an electron makes a transition from n=5 to n=2 in a hydrogen atom?

Answer 1

E = 2.18E-18(1/4 - 1/25)
That is E in J/atom
That x 6.02E23 = E in J/mol
Then convert to kJ.
1/4 = 1/n^2
1/25 = 1/5^2