Which of the following properties of an element is not variable?

Boron has two stable isotopes,$^{10}B$ $(19\%)$ and $^{11}B$ $(81\%)$. The atomic mass that should appear for boron in the periodic table is

The equivalent weight of a metal is $9$ and vapour density of its chloride is $59.25$. The atomic weight of the metal is:

Match the following substances in List-$I$ with their equivalent weights in List-$II$ ($M$ = Formula weight):

List-$I$ (Substance)	List-$II$ (Equivalent weight)
$A. Na_2CO_3$	$I. M/5$
$B. KMnO_4 / H^+$	$II. M/3$
$C. K_2Cr_2O_7 / H^+$	$III. M/2$
$D. KMnO_4 / H_2O$	$IV. M/6$

$A$ metal oxide is reduced by heating it in a stream of hydrogen. It is found that after complete reduction,$3.15 \, g$ of the oxide yielded $1.05 \, g$ of the metal. We may deduce that:

In the reaction $2Na_2S_2O_3 + I_2 \to Na_2S_4O_6 + 2NaI$,the equivalent weight of $Na_2S_2O_3$ (molecular weight = $M$) is equal to: