Answer
$1700\,J$
Work Step by Step
If the temperature of a monoatomic gas changes by $\Delta T$, its thermal energy changes by
$\Delta E_{th}= \frac{3}{2}nR\Delta T$.
$\implies \Delta E_{th}=\frac{3}{2}(0.50\,mol)(8.314\,Jmol^{-1}K^{-1})(270\,K)$
$=1700\,J$
The scientists must remove $1700\,J$ from the gas.
