Question

A given mass of oxygen at room temperature occupies a volume of 500.0 ml at 1.50 atm pressure. what pressure must be applied to compress the gas to a volume of only 150.0 ml? answer

Answer 1

Boyle's law gives the relationship between pressure of a gas and volume.
It states that the pressure of a gas is inversely proportional to volume of gas at a constant temperature.
PV = k
where P - pressure , V - volume and k - constant 
P1V1 = P2V2 
where parameters for the first instance are on the left side of the equation and parameters for the second instance are on the right side of the equation.
1.50 atm x 500.0 mL = P x 150.0 mL 
P = 5 atm
applied pressure should be 5 atm to compress the gas.