The $pH$ of a solution is $2$. If its $pH$ is to be raised to $4$,then the $[H^{+}]$ of the original solution has to be

Find the concentration of hydrogen ions in a solution with $pH = 5.4$ in $mol/L$.

At $45 \, ^oC$,the $pK_w$ of a neutral solution is $13.36$. The $pH$ of the solution will be .....

What is the dissociation constant of water at $25^{\circ}C$?

$2$. Given below are two statements:
Statement $I$: The changes in $pH$ with temperature are so small that we often ignore them.
Statement $II$: When the hydrogen ion concentration changes by a factor of $100$,the $pH$ changes by one unit.
In the light of the above statements,identify the correct answer from the options given below:

Observe the following solutions:
$I$. Black coffee
$II$. $0.2 \ M \ NaOH$
$III$. Lemon juice
$IV$. Lime water
$V$. Human saliva
$VI$. Tomato juice
The number of solutions having $pH$ range of $1-7$ and $7-14$,in the above list,is respectively.