How many moles of ammonia are needed to react completely with 3 moles of phosphoric acid?

The  moles of ammonia  that  are needed to react completely with 3  moles of  phosphoric acid  is 6 moles

    Explanation

Step 1:

write the a balanced  chemical reaction  that occur between ammonia and phosphoric acid.

3NH3 + H3PO4 → (NH4)3PO4

Step 2: use of mole ratio of NH3 :H3PO4  to calculate moles of  NH3

that is, NH3: H3PO4   is 3:1 therefore the moles of NH3 = 3moles x 3/1 = 9 moles

3 moles of NH3 was in excess therefore  the  mole of NH3 that  reacted = 9-3 = 6 moles