Question

2C2H6(g) + 7O2 -> 4CO2 (g) + 6 H2o (g)
What quantity of reactant remains after ignition of a mixture that contains 0.40 moles of C2H6 mixed with 1.60 moles of O2? ( assume maximum reaction according to the equation above)

Answer 1

This is a limiting reagent problem. Here is an example of such a problem. Just follow the steps to find the limiting reagent, then work backwards to find how much of the non-limiting reagent is used. Finally, subtract amount used from initial amount to determine amount remaining.
http://www.jiskha.com/science/chemistry/limiting_reagent_problem.html

Answer 2

.20 mol of o2