6.02 × 10^ 20 molecules of urea are present in 100 ~mL of its...

Given, number of molecules of urea $=6.02 \times 10^{20}$$\begin{aligned} & \therefore \text{ Number of moles }=\frac{6.02 \times 10^{20}}{N_A} \\ & =\frac{6.02 \times 10^{20}}{6.02 \times 10^{23}}=1 \times 10^{-3} \mathrm{~mol} \end{aligned}$Volume of the solution $=100 \mathrm{~mL}=\frac{100}{1000} \mathrm{~L}=0.1\mathrm{~L}$$Concentration of urea solution$$\begin{aligned} \left(\text{ in } \mathrm{mol} \mathrm{L}^{-1}\right) & =\frac{1 \times 10^{-3}}{0.1} \mathrm{mol} \mathrm{L}^{-1} \\ & =1 \times 10^{-2} \mathrm{~mol} \mathrm{~L}^{-1}=0.01 \mathrm{~mol} \mathrm{~L}^{-1} \end{aligned}$$