How Does Altitude Affect the Properties of Helium in a Balloon?

[Niles]

Homework Statement

I have a balloon on the ground, where it is 20 degrees celcius, and it has a volume of 5L (it is filled with helium). The pressure at the ground is 1 atmosphere. The balloon itself (without helium) weighs 0.1 kg.

I find the force that makes the balloon go up to be:

F_up = (m_air)*g = 58.96 N

F_down = (m_Helium + m_ballon) * g = 9.13 N

So the balloon will go up.

I can also find the number of Helium-moles from n = m/M, and I find that n = 207 moles.

The Attempt at a Solution

My questions are:

As the balloon goes up, the pressure goes down and so does the temperature. I want to know, which parametres are affected by this?

The density of the helium? Yes, because density is written as "rho" = M*p/(R*T), so this changes.

The volume of the helium? yes, this changes as well.

The number of helium-moles? No, this is constant too.

The mass of the helium? Yes, this gets smaller when the temperature goes down since "rho" = m/V.

The pressure inside the balloon? No, this stays the same.

Can you guys please confirm this?

 

[Niles]

- the pressure of the balloon is constant also, right?

 

[Moetasim]

Yes, your understanding is correct. As the balloon rises and the temperature and pressure decrease, the density and volume of the helium will change, but the number of moles will remain constant. This is because the ideal gas law, which states that PV = nRT, shows that the number of moles is directly proportional to the pressure and temperature. As the pressure and temperature decrease, the volume and density of the gas will also decrease. The pressure inside the balloon will remain constant because the balloon is sealed and the gas inside is in equilibrium.