A solution of sodium sulfate contains $$92 g$$  of $$N{a^ + }$$ ions per kilogram of water. The molality of $$N{a^ + }$$ ions in that solution in mol $$k{g^{ - 1}}$$  is :

Solution :
$$\eqalign{ & {\text{Number of moles in 92 g of N}}{{\text{a}}^ + } = \frac{{92}}{{23}} = 4\,moles \cr & {\text{Molality (m) = }}\frac{{{\text{Number}}\,{\text{of moles}}}}{{{\text{Mass of solvent (in kg)}}}} \cr & \therefore \,\,m = \frac{4}{1} = 4\,mol\,k{g^{ - 1}} \cr} $$