# dr. bob

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Use the data in Table 5.4 to calculate the partial pressure of He in dry air assuming that the total pressure is 1.0 atm. Assuming a temperature of 21°C, calculate the number of He atoms per cubic centimeter.
atoms/cm3

.00000524 - atmospheric composition of airnear sea level for He(info from table)

not a clue

• dr. bob -

What are the units here (for the composition of Helium at sea level)? percent? mols?

• dr. bob -

moles

• dr. bob -

In the Earth's atmosphere, the concentration of helium by volume (also called the "mole fraction") is 5.2 parts per million. At 1 atm and 21C, the molar density of air is
1 mole /24,100 cm^3. Multiple that by the mole fraction and Avogadro's number for the number of He atoms per cm^3.
I get 1.3*10^13.

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