Question

You have .900 mol of H atoms in the ground state. How much energy is required to transition all of the atoms from the ground state to n=4

Answer 1

energy required for 1 atom = -13.6 [1/nf^2 - 1/ni^2] eV

                    = -13.6 [1/4^2 -1/1^2] eV

                    = 12.75 eV

                     = 12.75*1.6*10^-19 J

                     =2.04*10^-18 J

number of atoms in 0.9 mol = 0.9*6.022*10^23 atoms = 5.4198*10^23 atoms

So,

total energy required = energy released by 1 atom * total number of atoms

                              = 2.04*10^-18 * 5.4198*10^23

                              = 1.1*10^6 J

Answer: 1.1*10^6 J