Question

If 21.6 grams of diphosphorus pentoxide and 8.2 grams of water combine to form phosphoric acid, how many grams of phosphoric acid must form

Answer 1

There is a common process for solving all problems like this. FIRST you MUST have a balanced chemical equation. For this reaction the equation is:

P2O5(s) + 3 H2O(l) --> 2 H3PO4

I'm sure you know that the balanced equation shows you the relationships between moles of one thing and moles of any other thing in the reaction.

Since you are given a mass of water, you must first convert that into moles:

8.2 g H2O / 18.0 g/mol = 0.45 moles H2O

Then, you use the coefficients in the equation to relate moles of H2O to moles of H3PO4:
0.45 moles H2O X (2 moles H3PO4 / 3 moles H2O) = 0.3 moles H3PO4

Finally, you convert the moles of H3PO4 into grams of H3PO4:

0.3 moles H3PO4 X 98.0 grams/mole = 24.9 grams H3PO4