Which one of the following statements regarding Henry's law is not correct ?

Consider the reversible isothermal expansion of an ideal gas in a closed system at two different temperatures T₁ and T₂ (T₁ < T₂). The correct graphical depiction of the dependence of work done (w) on the final volume (V) is :

The one that is extensively used as a piezoelectric material is :

A solution of sodium sulphate contains 92 g of Na⁺ ions per kilogram of water. The molality of Na⁺ ions in that solution in mol kg^$$-$$1 is :