posted by Brad on .
a balloon contains 30.0 L of helium gas at 103 kPa. What is the volume of the helium when the balloon rises to an altitude where the pressure is only 25.0 kPa?
You need to know how much the temperature changes. For an altitude rise by that much, atmospheric temperature typically drops from 288 K to 217 K. (I got that info from a standard atmosphere table). You could also assume an adiabatic expansion, in which case
T2/T1 = (P2/P1)^[(g-1)/g)]
where g = 5/3 for helium
That would give you T2 = 163 K. This probably won't happen, because outside air heat transfer will warm the balloon. So will sunlight, if there is any.
Assume P1 V1/T1 = P2 V2/T2 and solve for V2.
V2 = V1*(P1/P2)*(T2/T1)
If they expect you to assume T is constant, you need a better teacher or text.