Page 370 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
12 a Explain what is meant by an ideal gas. [2] b A cylinder contains 500 g of helium at a pressure of 5.0 × 105 Pa and at a temperature of 27 °C.
You may assume helium acts as an ideal gas. Calculate:
i the number of moles of helium the cylinder holds [1]
ii the number of atoms of helium the cylinder holds. [1]
c Calculate the volume of the cylinder. [3] d When the tap of the cylinder is opened for a short time a small amount of the helium escapes into
the atmosphere. As it does so, the temperature of the helium drops significantly.
i Explain why the temperature drops. [2]
ii Describe what happens to the average speed of the atoms of the escaped helium. [1]
(Relative atomic mass of helium = 4.)
13 A hot air balloon, its basket and passengers has a total mass of 450 kg. The inflated balloon holds 3000 m3 of air.
a Calculate the minimum force needed to lift the balloon off the ground. [1]
b Atmospheric pressure is 1.03 × 105 Pa and the density of air at this pressure is 1.23 kg m−3.
i Calculate the mass of 3000 m3 of air. [1]
ii Calculate the number of moles of air in the balloon. You may consider air at this pressure to act
as an ideal gas with a relative molecular mass of 29. [1]
c As the air is heated it expands and some of it is expelled through a vent at the top of the balloon.
Calculate the maximum mass of air that can remain in the balloon to give sufficient upthrust for the
balloon to just leave the ground. [2]
d Calculate the minimum temperature that the air inside the balloon must be heated to for the balloon to
just lift off the ground. [4]
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