To convert the mass of an atom from atomic mass units (amu) to grams, we can use the conversion factor:
1 amu = 1.66054 x 10^(-24) grams
Therefore, to convert the mass of an atom of phosphorus from amu to grams, we can multiply the given mass by the conversion factor:
30.9783 amu * (1.66054 x 10^(-24) grams/1 amu) = 5.13972 x 10^(-23) grams
So, the mass of an atom of phosphorus with a mass of 30.9783 amu is approximately 5.13972 x 10^(-23) grams.