Molar concentration $(M)$ of any solution $=$

Which of the following statements is correct?
$(1)$ Molarity is the number of moles of solute in one liter of solution.
$(2)$ The normality and molarity of a sodium carbonate solution are the same.
$(3)$ The number of moles of solute dissolved in $1000 \ g$ of solvent is defined as molality $(m)$.
$(4)$ The ratio of the mole fractions of solute and solvent is equal to the ratio of their respective moles.

Conc. $H_2SO_4$ has a density of $1.98 \ g/mL$ and is $98\%$ $H_2SO_4$ by weight. Its normality is ......... $N$.

The molarity of $20\% \,(W/W)$ solution of sulphuric acid is $2.55 \, M$. The density of the solution is $........... \, g \, cm^{-3}$.

$8 \ g$ of $NaOH$ is dissolved in $18 \ g$ of $H_2O$. Mole fraction of $NaOH$ in solution and molality (in $mol \ kg^{-1}$) of the solution respectively are

Calculate the molarity of a solution prepared by dissolving $7.1 \ g$ of sodium sulfate $(Na_2SO_4)$ in water to make $100 \ mL$ of solution. (in $M$)