The volume of a sphere is given by $\frac{4}{3}\pi r^3$, where $r$ is the radius. The mass is then given by the volume multiplied by the density $\rho$ of water, which is $1000 \, \mathrm{kg/m^3}$. So, the mass $m$ of the raindrop is
\[m = \frac{4}{3}\pi r^3
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