Question

Using the following equation

2C2H6 +7O2 -->4CO2 +6H2O

if 19 g C2H6 react with 115 g O2, what is the limiting reactant?

Answer 1

The balanced equation says that 2 moles C₂H₆ and 7 moles O₂ react together, i.e. in a ratio of 7:2 or 3.5 moles of O₂ to C₂H₆.

With molar masses 30.07 g/mol (C₂H₆) and 31.998 g/mol (O₂), the given quantities amount to

(19 g C₂H₆) × (1/30.07 mol/g) ≈ 0.63 mol C₂H₆

(115 g O₂) × (1/31.998 mol/g) ≈ 3.59 mol O₂

Now, 0.63/2 ≈ 0.32, and for every 0.32 mol C₂H₆ consumed, the reaction requires 7×0.32 ≈ 2.2 mol O₂. Then in order to consume all of the C₂H₆, the reaction would need 2×2.2 ≈ 4.4 mol O₂, which we don't have.

In other words, we have too much C₂H₆ and not enough O₂, so O₂ is the limiting reactant.