Answer
$1.6\times 10^6\; g/m^3$.
Work Step by Step
Mass of Neptune $m=1.02\times 10^{26}\;kg=1.02\times 10^{29}\;g$.
Radius of Neptune $r=2.48\times 10^4\;km=2.48\times 10^7\;m$.
Volume of Neptune
$V=\frac{4}{3}\pi r^3$
$V=\frac{4}{3}\pi (2.48\times 10^7\;m)^3$
$V=6.4\times 10^{22}\;m^3$
Density.
$=\frac{m}{V}$
Substitute all values.
$=\frac{1.02\times 10^{29}\;g}{6.4\times 10^{22}\;m^3}$
Simplify.
$=1.6\times 10^6\; g/m^3$.
