Which method is used to determine atomic mass accurately?

Suppose two elements $X$ and $Y$ combine to form two compounds $XY_{2}$ and $X_{2}Y_{3}$. If $0.05 \ mol$ of $XY_{2}$ weighs $5 \ g$ while $3.011 \times 10^{23}$ molecules of $X_{2}Y_{3}$ weighs $85 \ g$,then atomic masses of $X$ and $Y$ are respectively :

Heating $1.520 \ g$ of a metal hydroxide gives $0.995 \ g$ of the metal oxide. Calculate the equivalent weight of the metal.

The atomic mass of a metal is $27$ and its equivalent mass is $9$. What is the formula of its chloride?

$209 \ g$ of an element reacts with chlorine to form $315.5 \ g$ of its chloride. What is the weight (in $g$) of oxygen that reacts with $418 \ g$ of same element?
$(Cl = 35.5 \ u; O = 16 \ u)$

Sulphur forms the chlorides $S_2Cl_2$ and $SCl_2$. The equivalent mass of sulphur in $SCl_2$ is $....... \ g/eq$.