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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?

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2 answers

  1. a) 49.9 g

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  2. 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.

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