What is the sum of the mole fractions of all the components in a solution?

Sea water,which can be considered as a $6 \ M$ solution of $NaCl$,has a density of $2 \ g \ mL^{-1}$. The concentration of dissolved oxygen $(O_2)$ in sea water is $5.8 \ ppm$. Then the concentration of dissolved oxygen $(O_2)$ in sea water is $x \times 10^{-4} \ m$. $x = ............$ $(Nearest \ integer)$
Given: Molar mass of $NaCl$ is $58.5 \ g \ mol^{-1}$. Molar mass of $O_2$ is $32 \ g \ mol^{-1}$.

The quantity which changes with temperature is:

When $120 \ g$ of urea (molar mass $= 60 \ g/mol$) is dissolved in $1000 \ g$ of water,the density of the resulting solution is $1.15 \ g/mL$. The molarity of this solution is ........ $M$.

What is the concentration of nitrate ions if equal volumes of $0.1 \, M$ $AgNO_3$ and $0.1 \, M$ $NaCl$ are mixed together (in $, M$)?

If $50 \%$ of $1 \ M \ Na_2SO_4$ is dissociated in an aqueous solution of density $1.2 \ g \ mL^{-1}$,what is the molality of $Na^{+}$ ion in the solution?