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Helium gas contains molecules, each of which is a single helium atom. The mass of a helium atom is 4 atomic mass units.

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Question 1:
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What is the mass of 1 mole of helium gas?

Answer: mass (grams) = mass of molecule (atomic mass units) = 4 g

Joe places 2 grams of helium gas into a balloon.

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Question 2:
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How many moles of helium are in the balloon?

Answer: # moles = (mass of sample)/(mass of 1 mole) = (2 g)/(4 g) = 0.5 moles

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Question 3:
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How many molecules of helium are in the balloon?

Answer: # molecules = (# of moles)*(Avagadro's number) = (0.5) * (6.022 x 10^23) = 3.011 x 10^23

The balloon inflates to a diameter of 20 cm.

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Question 4:
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What is the volume of the balloon, in cubic meters?

Answer: 4*pi volume = ----- (radius)^3 3 4*pi*(0.10 m)^3 = --------------- = 4.19 x 10^(-3) m^3 3

The gas inside the balloon has a temperature of 25 degrees Celsius.

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Question 5:
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What is the pressure of the gas inside the balloon?

Answer: PV = nRT Note that temperature must be expressed in Kelvin: T(kelvin) = T(celsius) + 273 = 298 K Solve for pressure P: nRT (0.5 moles)*(8.31 J/mol*K)*(298 K) P = ------- = ---------------------------------- V 4.19 x 10^(-3) m^3 = 2.96 x 10^5 N/m^2 = 2.96 x 10^5 Pa

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This page maintained by Michael Richmond.
Last modified Jan 16, 1997.
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Copyright © Michael Richmond. This work is licensed under a Creative Commons License.