Which method is used to determine the molecular mass of a volatile substance?

The phosphate of a metal has the formula $MPO_4$. The formula of its nitrate will be

Isotope	Relative abundance $(\%)$	Atomic mass $(u)$
$^{12}C$	$98.8$	$12$
$^{13}C$	$1.18$	$13.1$
$^{14}C$	$0.02$	$14.1$

From the above data,what is the average molecular mass of $CH_4$ containing all isotopes of carbon,assuming hydrogen is only $^{1}_{1}H$? (Given that atomic mass of hydrogen $= 1.008 \ u$)

$A$ bivalent metal has an equivalent mass of $32$. The molecular mass of the metal nitrate is

If $A_1 \ g$ of an element gives $A_2 \ g$ of its oxide,then the equivalent mass of the element is .....

Which instrument is used to measure the atomic mass of an atom accurately?