When zeolite (hydrated sodium aluminium silicate) is treated with hard water,the sodium ions are exchanged with:

$A$ bag contains $n$ white and $n$ black balls. Pairs of balls are drawn at random without replacement successively,until the bag is empty. If the number of ways in which each pair consists of one white and one black ball is $14400$,then $n$ is equal to

The number of structural isomers for $C_6H_{14}$ is

During the electrolysis of cryolite,aluminium and fluorine are formed in ........ molar ratio :

The compound formed when gypsum is dissolved in aqueous ammonium sulphate solution is:

The probability distribution of a random variable $X$ is given below:

$X = x$	$0$	$1$	$2$	$3$
$P(X = x)$	$\frac{1}{10}$	$\frac{2}{10}$	$\frac{3}{10}$	$\frac{4}{10}$

Then the variance of $X$ is