This question relates to Avogadro's Number. Remember that Avogadro's Number is 6.023 x 1023. What this means is that per 1 mol of a compound, there will be 6.023 x 1023 atoms.
With that in mind, we know that the molar mass of sodium (Na) is 23 grams/mol. We can take this a step further now, using Avogadro's number, and say that there are 23 grams of sodium/6.023x1023 atoms. That will be the conversion factor for this calculation.
So as follows:
2.0x10 = 20
20 atoms of sodium = (1 mol of sodium/6.023x10^23 atoms of sodium) =
3.3x10^-23mols of sodium = (23 grams of sodium/1 mol of sodium) = 7.6x10^-22 grams of sodium.
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