Use the periodic table to determine how many atoms of phosphorus (p) are in a sample that has a mass of 172.90 g.

The atomic mass of Phosphorous is 31. the molar mass is numerically equivalent to the atomic mass.
the molar mass of P is 31 g/mol 
1 mol of P is made of 6.022 x 10²³ atoms of P.
this means that 31 g is made of 6.022 x 10²³ atoms of P.
therefore in 172.90 g there are - 6.022 x 10²³ / 31 x 172.90 = 3.359 x 10²⁴ atoms
there are 3.359 x 10²⁴ atoms of P