Which method is used to determine atomic mass accurately?

The average molecular weight of the air which has $21 \%$ of $O_2$ and $79 \%$ of $N_2$,is

Suppose the elements $X$ and $Y$ combine to form two compounds $XY_2$ and $X_3Y_2.$ When $0.1 \ mol$ of $XY_2$ weighs $10 \ g$ and $0.05 \ mol$ of $X_3Y_2$ weighs $9 \ g,$ the atomic weights of $X$ and $Y$ are

The mass of $500 \, cm^3$ of a gas at $STP$ is $0.581 \, g$. If the gas is a hydrocarbon,what is the gas?

$3.011 \times 10^{22}$ atoms of an element weigh $1.15 \ g$. The atomic mass of the element is: (in $amu$)

Complete reaction of $2.0 \, g$ of calcium (at. wt. $= 40$) with excess $HCl$ produces $1.125 \, L$ of $H_2$ gas. Complete reaction of the same quantity of another metal $M$ with excess $HCl$ produces $1.85 \, L$ of $H_2$ gas under identical conditions. The equivalent weight of $M$ is closest to