A typical value for the atmospheric pressure on Earth is 101 kPa. The surface area of the Earth A, is related to its mean radius R by the expression A = 4piR^2, where R has the value 6400 km.

Calculate the number of molecules in the atmosphere, assuming that the mass of a mole of air is 30 g.

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The pressure on the earth is applied by the weight of air in the atmosphere.

`P = F/A`

`P = 101kpa`

`A = 4piR^2 = 4xxpixx(6400xx10^3)^2 = 5.147xx10^14m^2`

`F = 1.01xx10^5xx5.147xx10^14 = 5.2xx10^19`

If the mass of air is m the;

`F = mg`

`5.2xx10^19 = mxx9.81`

`m = 5.3xx10^18kg`

It is given that mass of 1 mole of air is 30g.

Amount of moles in air `= (5.3xx10^18)/0.03 = 1.77xx10^20 mol`

According to Avogadro 1mol of substance contain `6.022xx10^23 ` atoms.

Atoms in air `= 6.022xx10^23 xx1.77xx10^20 = 1.065xx10^44`

*So there is `1.065xx10^44` atoms in air.*

*Assumption*

*Gravitational acceleration is constant throughout the atmosphere.*

**Sources:**

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