Molarity Questions

Concentrated aqueous sulphuric acid is $98\%{\text{ H}}_2{\text{SO}}_4$ by mass and has a density of $1.80{\text{ g mL}}^{-1}$. The volume of acid required to make one litre of $0.1{\text{ M H}}_2{\text{SO}}_4$ solution is:

$25.3\text{g}$ of sodium carbonate, ${\text{Na}}_2{\text{CO}}_3$ is dissolved in enough water to make $250\text{mL}$ of solution. If sodium carbonate dissociates completely, molar concentration of sodium ion, ${\text{Na}}^{+}$ and carbonate ions, ${\text{CO}}_3^{2-}$ are respectively: (Molar mass of ${\text{Na}}_2{\text{CO}}_3=106{\text{g mol}}^{-1}$)

$6.02\times 10^{20}$ molecules of urea are present in $100\text{mL}$ of its solution. The concentration of the solution is: