J.R. S. answered 02/08/21
Ph.D. University Professor with 10+ years Tutoring Experience
Assuming the compound dissociates completely, you would get 4 particles (ions) from each particle (molecule) of (NH4)3PO4 that was present. (NH4)3PO4 ==> 3NH4+ + PO43- (4 particles).
If you have 0.573 m solution, that would be 0.573 moles of (NH4)3PO4 per kg of solvent. The molality of ions would be 4 x 0.573 m = 2.29 m