First you should find no of mole.
a mole is a measurement of the number of number of grams in which there are 6.02 x 10^23 atoms/molecules of a substance.
1.468 moles of NH3 in 25 grams.
(25/17.0306) 1.468 x (6.02 x 10^23) = 8.837 x 10^23 molecules.
hope this help
