Use Boyle's Law: P₁V₁ = P₂V₂. Since pressure and volume are inversely related and we're increasing the pressure, we should expect the new volume to be less than 2.60 liters.
Here, P₁ = 1.00 atm, V₁ = 2.60 liters, and P₂ = 3.50 atm; we want to find the new volume, V₂, at P₂. We can modify the equation to solve for V₂:
V₂ = P₁V₁/P₂ = (1.00 atm)(2.60 L)/(3.50 L) = 0.714 L
So the volume of the gas at a pressure of 3.50 atm will be 0.743 L.
