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.
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?
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