The energy of electron in first Bohr's orbit of H-atom is -13.6 eV. What will be its potential energy in n = 4th orbit.

Energy of nth orbit of H-atom

                         $=-\frac{2{\mathrm{\pi }}^2{\mathrm{me}}^4{\mathrm{k}}^2}{{\mathrm{h}}^2}\times \mathrm{\hspace{0.17em}\hspace{0.17em}}\frac{1}{{\mathrm{n}}^2}$

Energy of  first Bohr's orbit of H-atom

                                       $=-\frac{2{\mathrm{\pi }}^2{\mathrm{me}}^4{\mathrm{k}}^2}{{\mathrm{h}}^2}$

                                             =-13.6 eV (given)

Energy of fourth Bohr's orbit of H-atom

$=-\frac{2{\mathrm{\pi }}^2{\mathrm{me}}^4{\mathrm{k}}^2}{{\mathrm{h}}^2}\times \mathrm{\hspace{0.17em}\hspace{0.17em}}\frac{1}{4^2}$

$=13.6\times \frac{1}{16}\mathrm{eV}=-0.85\mathrm{eV}$   (or)  ${\mathrm{E}}_{\mathrm{n}}=\frac{{\mathrm{E}}_1}{{\mathrm{n}}^2}\times {\mathrm{Z}}^2=-\frac{13.6}{2^2}\left(1^2\right)=-0.85\mathrm{eV}$

       PE of electron in nth orbit = $2\times {\mathrm{E}}_{\mathrm{n}}$

So P.E. of electron in 4th orbit = $2\times \left(-0.85\right)=-1.70eV$