The new pressure is 1.714 atm
calculation
The new pressure is calculated using the Boyle's law formula
that is P₁V₁= P₂V₂
where P₁=0.857 atm
V₁ = 1.0 L
P₂ = ?
V₂= 0.5 L
make P2 the subject of the formula by diving both side by V₂
P₂ = P₁V₁ / V₂
P₂ = (0.857 atm x 1.0 L)/ 0.5 L = 1.714 atm