if 20.2g of NO and 13.8g of O2 are used to form NO2, how many moles of excess reactant will be left over

0.095 moles of O₂ are left over.

Explanation:

First of all, state the balanced reaction:

2NO + O₂ → 2NO₂

We determine moles of each reactant:

20.2 g . 1mol / 30g =  0.673 moles of NO

13.8g . 1mol / 32g = 0.431 moles of oxygen

Oxygen is the excess reactant. Let's see.

For 2 moles of NO I need 1 mol of O₂

Then, for 0.673 moles of NO I may use (0.673 .1) /2 = 0.336 moles

I have 0.431 moles of O₂ and I only need 0.336 mol. According to reaction, stoichiometry is 2:1.

In conclussion, the moles of excess reactant that will be left over:

0.431 - 0.336 = 0.095 moles

if you mean hydrogen and oxygen then water is the product