Chemical reaction balancing problem

Question

4.800 g of H2 was mixed and ignited with 36.40 g of O2.

1. How much and which substances will be present in the reaction flask when the reaction is complete?
2. Prove that the total mass of all the substances after the reaction is equal to the total mass of the substances before the reaction.

Note: take into consideration a possible excess

Answer 1

The reaction of H₂(g) and O₂(g) is as follow:

2H₂(g) + O₂(g) ==> 2H₂O(l) .. balanced equation

4.800 g H₂ x 1 mol H₂ / 2.02 g = 2.376 mols H₂

36.40 g O₂ x 1 mol O₂ / 32 g = 1.1375 mols O₂

Because the balanced equation shows 2 mols H₂ per every 1 mol O₂, we can see that O₂ is limiting (will run out first). Thus, the mols of O₂ will determine mols of H₂O formed, and H₂ will be left over in excess.

(1). mols H₂O formed: 1.1375 mols O₂ x 2 mols H₂O / mol O₂ = 2.275 mols H₂O will form

The flask will also contain 0.101 mols H₂(g) left over in excess.

(2). total mass of reactants before reaction = 4.8000 g + 36.40 g = 41.2 g

total mass of all substances after reaction:

2.275 mols H₂O x 18 g / mol = 40.95 g

0.101 mols H₂(g) left over x 2.02 g / mol = 0.204 g H₂(g)

Total mass = 40.95 g + 0.20 g = 41.15 g = 41. 2 g which agrees with mass of material before reaction