Bell metal is an alloy of which of the following?

$A$ compound $(A)$ is used in the preparation of washing soda to recover ammonia in the Solvay process. When $CO_2$ is bubbled through an aqueous solution of $(A)$,the solution turns milky. It is also used in whitewashing due to its disinfectant nature. What is the chemical formula of $(A)$?

Light of wavelength $4000 \, \mathring A$ is incident on a sodium surface for which the threshold wavelength of photoelectrons is $5420 \, \mathring A$. The work function of sodium is .......... $eV$.

With an increase in atomic number,the melting point of alkali metals .......

Consider the following gaseous equilibria with equilibrium constants $K_{1}$ and $K_{2}$ respectively.
$SO_{2(g)} + \frac{1}{2} O_{2(g)} \rightleftharpoons SO_{3(g)}$
$2 SO_{3(g)} \rightleftharpoons 2 SO_{2(g)} + O_{2(g)}$
The equilibrium constants are related as

If $I_n = \int \sin^n x \, dx$,then $n I_n - (n-1) I_{n-2}$ equals