Which of the following is an integral multiple of $h/2\pi$ in Bohr's model of a hydrogen atom?

If the speed of an electron of a hydrogen atom in the ground state is $2.2 \times 10^6 \ m/s$,then its speed in the third excited state will be

If the total energy of an electron in an orbit is positive,then

The electric potential between a proton and an electron is given by $V = V_0 \ln(r/r_0)$,where $r_0$ is a constant. Assuming Bohr's model to be applicable,find the variation of $r_n$ with $n$,where $n$ is the principal quantum number.

Consider the spectral line resulting from the transition $n = 2 \rightarrow n = 1$ in the atoms and ions given below. The shortest wavelength is produced by:

The number of revolutions per second made by an electron in the first Bohr orbit of a hydrogen atom is of the order of