HardPhysicsHigh School (11–12)Heat & Thermodynamics › Kinetic Theory of GasesHeat & Thermodynamics › Temperature and Thermal ExpansionNumerical
A party balloon holds 12 L of helium at 101.3 kPa and 300 K…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A party balloon holds L of helium at kPa and K. The rubber has a mass of g. Take g/mol for helium and g/mol for air, and m/s². Find the mass of the helium, the mass of the air the balloon pushes aside, and the largest extra mass the balloon can hold up.
Need a reminder? The key ideaShow
For an ideal gas, with in kelvin and the absolute pressure:
For a fixed amount of gas, . The density of a gas is .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Plan. Both the helium and the air it pushes aside fill L at the same and . So they have the same number of moles.
Step 1. Moles. .
Step 2. Helium. g.
Step 3. Air. g.
Step 4. The extra mass. The air pushes up with the weight of g. The balloon itself has g. So it can hold up g more. That is a net upward force of about N.
Answer: g of helium, g of air, and g of extra mass.
What if? Fill it with hot air instead. Air at K has of the density of air at K. Its gas would weigh g, so the balloon could not even hold up its own rubber.
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