The density of a mixture of $He$ and $O_2$ gases at $27\,^{\circ}C$ and $760\, torr$ pressure is $0.543\, g\, dm^{-3}$. Calculate the percentage by mass of $He$ in the mixture.

$A$ $10 \ g$ mixture of hydrogen and helium is contained in a vessel of capacity $0.0125 \ m^{3}$ at $6 \ bar$ and $27^{\circ}C$. The mass of helium in the mixture is $g$. (nearest integer) Given : $R = 8.3 \ J \ K^{-1} \ mol^{-1}$ (Atomic masses of $H$ and $He$ are $1 \ u$ and $4 \ u$,respectively)

Why does a sharp glass edge become smooth on heating it up to its melting point in a flame? Explain which property of liquids is responsible for this phenomenon.

Assertion : Wet air is heavier than dry air.
Reason : The density of dry air is more than density of water.

Which one of the following statements is false?

Based on the given figure,the number of correct statement/s is/are $........$.
$A.$ Surface tension is the outcome of equal attractive and repulsion forces acting on the liquid molecule in bulk.
$B.$ Surface tension is due to uneven forces acting on the molecules present on the surface.
$C.$ The molecule in the bulk can never come to the liquid surface.
$D.$ The molecules on the surface are responsible for vapour pressure if the system is a closed system.