Which one of the following corresponds to the wavelength of line spectrum of H atom in its Balmer series ? ( $R=$ Rydberg constant)

A 100 W bulb emits light of wavelength

$x \mathop {\rm{A}}\limits^{\rm{o}} $. What is the value of $x$, if the number of photons emitted is $2.0 \times 10^{20} \mathrm{~s}^{-1}$ ?

$$ \left(h=6.63 \times 10^{-34} \mathrm{Js}, 1 \mathrm{~W}=1 \mathrm{Js}^{-1}\right) $$

The ratio of the difference in energy between the first and second Bohr orbits to that between the second and third orbit is

In the reaction I and II the covalencies of Be and Al in $X$ and $Y$ are respectively

I. $\mathrm{Be}(\mathrm{OH})_2+\underset{\text { (Excess) }}{\mathrm{NaOH}} \longrightarrow X$

II. $\mathrm{Al}(\mathrm{OH})_3+\underset{\text { (Excess) }}{\mathrm{NaOH}} \longrightarrow Y$