The volume usually chosen is that occupied by a mole of the gas {its molecular weight in grams}, which is 22.4 liters at standard temperature and pressure. The number of molecules under such conditions later became known as Avogadro's number. Determining it precisely was, and still is, rather difficult. A current estimate is approximately 6.02214 × 1023. {This is a big number: that many unpopped popcorn kernels when spread across the United States would cover the country nine miles deep.}29
( Walter Isaacson )
[ Einstein: His Life and ]
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