The ideal gas law predicts gas behavior including the relationships between the number of moles, volume, pressure, and temperature. Predict the changes in volume of a helium-filled balloon at different temperatures. When you took the balloon outside on a cold day, the helium atoms inside move slower and its volume decreased because the atoms didn't exert as much pressure on the balloon walls. Calculate the volume in L of a balloon filled with 0.200 moles of helium at 1.00 atm at 0.00°C.