Copyright © Michael Richmond.
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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.
Question 1: 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.
Question 2: 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
Question 3: 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.
Question 4: 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.
Question 5: 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
This page maintained by Michael Richmond. Last modified Jan 16, 1997.
Copyright © Michael Richmond.
This work is licensed under a Creative Commons License.