Helium as a gas has density 0.164 kg/m^3 at room temperature and 1 atmosphere. As a boiling liquid, its density is 125 kg/m^3 (which incidentally is much less than water, 1000 kg/m^3).
1 cup of liquid helium therefore has a mass of 0.0296 kg. The same mass as a gas would have volume 0.180 m^3. Approximate a party balloon as a 10.5 inch diamater sphere, so it has volume 0.01 cubic meter. Then, 1 cup of liquid helium is enough gas to fill 18 balloons. This assumes the pressure inside a balloon is atmospheric pressure, which is false (but might still be close enough) because the tension of the latex squeezing the gas inside would increase its pressure.
Inspiration was, how much do we have to worry about wasting highly non-renewable helium on party balloons? I'm guessing typical cryogenic applications, e.g., MRI, use gallons of liquid helium.
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