The Bohr orbit radius for the hydrogen atom $(n = 1)$ is approximately $0.530 \ \mathring{A}$. The radius for the first excited state $(n = 2)$ is ............. $\mathring{A}$.

Earthworms have no skeleton,but during burrowing the anterior end becomes turgid and acts as a hydraulic skeleton. It is due to

Which of the following is a buffer solution?

The oxidation state of $Xe$ in $XeO_3$ and the bond angle in it respectively are:

For a first order reaction at $27^{\circ} C$,the ratio of time required for $75 \%$ completion to $25 \%$ completion of reaction is

The cathode of a photoelectric cell is changed such that the work function changes from $W_1$ to $W_2$ $(W_2 > W_1)$. If the current before and after the change are $I_1$ and $I_2$ respectively,with all other conditions remaining unchanged,then (assuming $h\nu > W_2$):