Which is the limiting reactant when 5.00g of H2 and 10.00g of O2 react and form water?

Explanation:

The reaction expression is given as:

           2H₂  +   O₂    →    2H₂O

The limiting reactant is the reactant in short short supply in the given reaction.

To find the limiting reactant we use the number of moles.

    Mass of H₂  = 5g

    Mass of O₂   = 10g

  Number of moles  =  

       Molar mass of H₂   = 2(1) = 2g/mol

       Molar mass of O₂  = 2(16)  = 32g/mol

Number of moles  H₂ =   = 0.4mole

Number of moles O₂ =   = 0.3mole

   From the balanced reaction equation:

           2 mole of H₂ reacts with 1 mole of O₂

          0.4 mole of H₂ reacts with  0.2 mole of O₂

But we are given 0.3 moles of O₂, so the limiting reactant is H₂ because oxygen gas is in excess

if you're asking what comes out, it's steam.