With handwritten solutions

A sample of oxygen, at 250C, occupies a volume of 500 mL at 1.50 atm pressure. What pressure must be applied to compress the gas to a volume of 150 mL, with no temperature change? Calculate the number of moles of helium in a 'l.00-L balloon at 270C '. and 100 atm of pressure. If 0.55 g of a gas dissolves in 1.0 L of water at 2 atm of pressure, how . much will dissolve at 4.5 atm