Density of water at room temperature is 1 g/ml. How many molecules are there in a drop of water, if its volume is 0.05 ml ?

Density of water at room temperature = 1 g/ml

$\begin{array}{l}\therefore 1\mathrm{ml}\mathrm{contain}\frac{1}{18}\mathrm{moles}\mathrm{of}{\mathrm{H}}_2\mathrm{O}\\ 0.05\mathrm{ml}\mathrm{will}\mathrm{contain}0.05\times \frac{1}{18}\mathrm{moles}\\ \therefore 1\mathrm{mole}=6.022\times {10}^{23}\mathrm{molecules}\\ \mathrm{Total}\mathrm{number}\mathrm{of}\mathrm{molecules}\mathrm{in}\mathrm{a}\mathrm{drop}\mathrm{of}\mathrm{water}\\ =6.023\times {10}^{23}\times 0.05\times \frac{1}{18}=1.67\times {10}^{21}\end{array}$