[blackjack18]

A sealed balloon is filled with 1.00 L of He gas at 23 deg C and 1.00 atm. The balloon rises to a point in the atmosphere where the pressure is 220 torr and the temperature is -31 deg C. What is the final volume of the balloon after it reaches an altitude where the pressure is 220 torr?

If I should use the Ideal Gas Law, Pv= nRT, how do I find the number of moles?
     

[nj_bartel]

In the first part of the question, you're given volume, temperature, and pressure (3/4 variables).  That's enough to solve the equation.  R is a constant (make sure you choose the right one).  P is typically in atmospheres, V in liters, n in moles, and T in kelvin.

You can assume that as the baloon travels upward, you aren't losing any helium.

[Borek]

Note, that you don't need moles to solve this question.

p₁V₁/T₁ = p₂V₂/T₂

[blackjack18]

so is the answer 0.0036?

[Mitch]

Make sure you use Kelvin in the problem.

[blackjack18]

I did...

[sjb]

And converted the pressure to the same units in both samples?