The volume of a gas is 450 ml when its pressure is 1.00 atm. if the temperature of the gas does not change, what is the pressure when its volume is changed to 2.00 l?

To solve this we assume that the gas is an ideal gas. Then, we can use the ideal gas equation which is expressed as PV = nRT. At a constant temperature and number of moles of the gas the product of PV is equal to some constant. At another set of condition of temperature, the constant is still the same. Calculations are as follows:

P1V1 =P2V2

P2 = P1V1/V2

P2 = 1 atm x 450 mL / 2000 mL

P2 = 0.225 atm

if you're asking what comes out, it's steam.