If 1 mL of water contains 20 drops then number of molecules in a drop of water is

(i) $\because$For water density = 1 g/ml

$\therefore$1000 mL = 1000 g

(ii) $\because$Number of moles of water in 1000 g (n) $=\frac{\mathrm{w}}{\mathrm{m}}\text{ (molar mass) }$

$\mathrm{n}=\frac{1000}{18}=55.56\mathrm{moles}$

$\therefore$Number of moles in I mL of water (n) = $\frac{55.6}{1000}$ 

                                                     (n) = 0.0556moles

(iii) $\because 1{\text{ mL of H}}_2\text{O contain 20 drops}$

$\therefore \text{\hspace{0.17em}\hspace{0.17em} Number of moles of }{\mathrm{H}}_2\mathrm{O}\text{ in one drop }=\frac{{\displaystyle 0.0556}}{{\displaystyle 20}}$

                                                         (n)Pet drop = 0.0027

(iv) $\because \mathrm{n}=\frac{\mathrm{N}}{{\mathrm{N}}_{\mathrm{A}}}=\frac{\mathrm{N}}{6.023\times {10}^{23}}$

$\mathrm{N}=\left(\mathrm{n}\times {\mathrm{N}}_{\mathrm{A}}=0.002\right)\times 6.023\times {10}^{23}$

$\mathrm{N}=0.0162\times {10}^{23}=0.62\times {10}^{21}$