Answer to Question #22977 in Physical Chemistry for Trish

Question

When methane is burned with oxygen, the products are carbon dioxide and water. If you produce 9 grams of water and 11 grams of carbon dioxide from 16 grams of oxygen, how many grams of methane were needed for the reaction?

Answer 1

First you need to write a balanced chemical equation. You are given thatmethane is burned, meaning a combustion reaction in which carbon dioxide and water are released.

Unbalanced: CH₄ + O₂ ---> CO₂ + H₂O
Balanced: CH₄ + 2O₂ ---> CO₂ + 2H₂O

Givens:
X grams CH₄ (Molecular mass 16.0 grams)
9 grams H₂O (Molecular mass 18.0 grams)
11 grams CO₂ (Molecular mass 44.0 grams)
Mole ratio: 1:2:1:2 (CH₄:O₂:CO₂:H₂O)

Then you need to find which of the reactants are the limiting reactant and which is the excess reactant. The limiting reactant is what you will base the rest of the problem on. To do this, you convert each measurement to moles from grams.

11 g CO₂ / (44.0 g) = 0.25 moles CO₂

9 g H₂O / (18.0 g) = 0.5 moles

n of CH₄ = n of CO₂ = n of H₂O /2 = 0.25 moles

m of CH₄ = n* M_w = 0.25 * 16.0 = 4 g