What volume will 1.25 liters of carbon dioxide occupy at 50.0°c if the pressure changes from 0.865 atm to 0.820 atm?

To solve this we assume that the gas inside the balloon 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

V2 = P1V1 / P2

V2 = 0.865 x 1.25 / 0.820

V2 = 1.32 L