first, is to solve the molar mass of the herbicide, with a formula
C3H8NO5P
C3 = 12 ( 3 ) = 36
H8 = 1 ( 8 ) = 8
N = 14
O5 = 16 ( 5 ) = 80
P = 31
so in total the molar mass of C3H8NO5P is 36 + 8 + 14 + 80 + 31 = 169 g/mol
number of moles = 783.5 g ( 1 mol / 169 g)
number of moles = 4.64 mol C3H8NO5P