Suppose that 4.9 mol no2 and 0.40 mol h2o combine and react completely. which reactant is in excess?

Answer is: NO₂ (nitrogen (IV) oxide) is in excess.

Balanced chemical reaction: 3NO₂(g) + H₂O(l) → 2HNO₃(l) + NO(g).
n(NO₂) = 4.9 mol.
n(H₂O) = 0.4 mol.
From chemical reaction: n(NO₂) :(H₂O) = 1 : 3.
For 0.4 mole of water, we need: 0.4 mol : n(NO₂) = 1 : 3; n(NO₂) = 1.2 mol.
Because we have more than 1.2 moles of nitrogen(IV) oxide, this molecule is in excess.