#### Answer

0.50 atm

#### Work Step by Step

$V_{1}=1.0\,L$
$P_{1}=2.0\,atm$
$V_{2}=4.0\,L$
Temperature and amount of gas are constant.
Boyle's law states that
$P_{1}V_{1}=P_{2}V_{2}$ (unchanging T and n)
$\implies P_{2}=\frac{P_{1}V_{1}}{V_{2}}=\frac{2.0\,atm\times1.0\,L}{4.0\,L}$
$=0.50\,atm$