What is the reactants start for H2 + O2 --> H2O

H2 + O2

Explanation:

H2 + O2 --> H2O

The items on the left side is the reactants.  The arrow means yields, and the item the arrow is pointing at is the product.

.081 g of O2

Explanation:

4Cr + 3O2 > 2Cr2O3

.175 g Cr x [1 mole / 52.0 g] x [2 moles Cr2O3 / 4 moles Cr] x [152 g / 1 mole] = .256 g of Cr2O3

.175 g Cr x [1 mole / 52.0 g] x [3 moles O2 / 4 moles Cr] x [32 g / 1 mole] = .081 g of O2