What is the kinetic energy of an electron in the second excited state of $He^{+}$? (Energy of an electron in the first shell of $H = -13.6 \text{ eV}$)

Two nuclei have their mass numbers in the ratio of $1:3$. The ratio of their densities would be

All of the following substances react with water. Which pair among them will produce the same gaseous product upon reaction with water?

Let $A$ and $B$ be two symmetric matrices of the same order. Then,the matrix $AB - BA$ is

The radius of the first orbit of hydrogen is $r_{H}$,and the energy in the ground state is $-13.6 \text{ eV}$. Considering a $\mu^{-}$-particle with a mass $207 m_e$ revolving around a proton as in a hydrogen atom,the energy and radius of the proton and $\mu^{-}$-combination respectively in the first orbit are (assume the nucleus to be stationary):

If ${}^n C_{r-1}=330$,${}^n C_r=462$,and ${}^n C_{r+1}=462$,then $r$ is equal to