The density of water at room temperature is $1.0g/ml$. How many molecules are there in a drop of water if its volume is $0.05ml?$

We have, density = $\frac{mass}{volume}$.

mass = density*volume

So, mass of one drop of water = $1.0\ast 0.05=0.05g$.

Number of water molecules in $18g$ of water = $N_A$.

So, number of molecules in $0.05g$ = $\frac{N_A}{18}\ast 0.05=0.0027N_A$.