Question

27.8 grams of diphosphorus pentoxide are allowed to react with 9.22 grams of water
What is the maximum amount of phosphoric acid that can be formed? (Grams)
What is the formula for the limiting reagent?
What amount of the excess reagent remains after the reaction is complete? (Grams)

Answer 1

P2O5 + 3H2O --> 2H3PO4
mols P2O5 = g/molar mass = 27.8/141.94 (but check that) = 0.196
mols H2O = 9.22/18.02 = 0.512
From the reaction you know that 1 mol P2O5 will require 3 mols H2O; therefore, 0.196 mols H3PO4 will require 3*0.196 or 0.588 mols H2O and you have that much H2O so H2O is the limiting reagent (LR) and P2O5 is the ER (excess reagent).
Using the LR, 0.512 mols H2O x (2 mols H3PO4/3 mols H2O) = 0.512 x 2/3 = 0.314 mols H2PO4. Convert to grams by g = mols x molar mass = ? g H3PO4 = maximum H3PO4 that can be obtained. Then
Amount H3PO4 initially - max amount obtained = amount ER remaining.
Post your work if you get stuck