The balloon crowd

A spherical balloon of radius $$10~\mbox{cm}$$ is filled with air of density $$1.23~\mbox{kg}/\mbox{m}^3$$. What's the order of magnitude for the number of air molecules inside of the balloon?

Details and assumptions

• An air molecule weighs on average $$4.81 \times 10^{-23}~\mbox{kg}$$.
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