Question

Phosphorus pentachloride, PCl5, reacts with water to form
phosphoric acid, H3PO4, and hydrochloric acid, HCl.
PCl5(s) + 4H2O() → H3PO4(aq) + 5HCl(aq)
(a) What mass of PCl5 is needed to react with an excess quantity of H2O
to produce 23.5 g of H3PO4?
(b) How many molecules of H2O are needed to react with 3.87 g
of PCl5?

Answer 1

PCl5(s) + 4H2O() → H3PO4(aq) + 5HCl(aq)

mols H2O = grams/molar mass = 23.5/18 = ?
Use the coefficients in the balanced equation to convert mols H2O to mols PCl5. Then convert mols PCl5 to g. g = mols x molar mass.

Part b.
Done the same way to find mols H2O then convert to # molecules remembering that 1 mol contains 6.02E23 molecules.

Answer 2

a) 49.9 g