The number of moles is
That's a Chemistry question and you need to use the ideal gas law to find the number of moles
Here, we have a ballon with a radius of
The volume is given by
Therefore, by the Rule of Three :
For the temperature,
For the mass,
The molar mass of
The number of moles of helium will be equal to 1.1 and the mass of helium will be 4.4 g.
SIDE NOTE Small correction - helium cannot exist as a diatomic molecule, so the correct notation is
There's nothing wrong with the method used in the other answer, but I want to show you how you can get the number of moles of helium without doing that many conversions.
You that your balloon has a radius of 18 cm. You can determine its volume in
Use the fact that
Now use the ideal gas law equation to solve for
To get the mass of helium, use its molar mass, which expresses the mass of 1 mole of helium