Energy required to excite hydrogen atom from ground state to the excited state
= Efinal = Einitial
= -1.51 - (-13.6) = 12 .09 eV
i.e. hydrogen atoms would be excited up to third energy level (i.e. r : 3) / second excited state. [Note If the student just writes gaseous hydrogen is made up of the molecule or just writes the formula for En, award this mark.]
Alternatively
[Note : Also accept the answer given in terms of R only]