Limiting Reactants Problem?

Question

Given the following chemical reactions: 2Al + 2H(3)PO(4) -> 2AlPO(4) + 3H(2)

If 35g of Aluminum reacts with 50g of phosphoric acid, what is the limiting reagent?

what mass of hydrogen gas will be produced by the reactants in part a?

Answer 1

First,determine the number of moles of your reactants from the start...

moles of aluminum = 35 g / 26.98 g/mole  = 1.297 moles

moles of H3PO4 = 50 g / 98 g/mole = 0.51 moles

then from the reaction you can see that 2 moles of Al reacts with 2 moles of H3PO4...so the ratio is 1:1...and since phosphoric acid is only 0.51 moles, it is the limiting reagent

As for the H2 gas produced you can see from the reaction the ratio of phosphoric acid to H2 gas is 2:3

therefore:

moles of H2 = moles of H3PO4 (moles of H2 in the equation/moles of H3PO4 in the equation)

moles of H2 = 0.51 moles (3/2)

moles of H2 = 0.765 moles or 0.77 moles considering significant figures