Answer
$6.0\times10^{17}\ km$
Work Step by Step
1. Compute the length of a stack of Avogadro's number of pennies in meters.
$6.022\times10^{23} \times (1.0\times10^{-3})=6.0\times10^{20}\ m$
2. Convert this length into kilometers.
$6.0\times10^{20}\times \frac{1\ km}{1000\ m}= 6.0\times10^{17}\ km$
