Chemistry Stoichiometry

Question

How many milligrams of oxygen are required to create 5.14 liters of carbon dioxide gas at STP from the complete combustion of methane (CH₄)? Must be in scientific notation

Answer 1

The first thing you MUST do is write the correctly balanced equation for the reaction:

CH₄ + 2O₂ ==> CO₂ + 2H₂O ... balanced equation for combustion of methane

At STP (standard temperature and pressure), 1 mole of any ideal gas = 22.4 liters (memorize this)

Using dimensional analysis, we can find the moles O₂ needed...

5.14 L CO₂ x 1 mol CO₂/22.4 L x 2 mol O₂ / mole CO₂ = 0.459 moles O₂ needed

Converting moles O₂ to g O₂ and then to mg O₂, we have...

0.459 moles O₂ x 32 g O₂/mol O₂ x 1000 mg/g = 14656 mg O₂ = 14700 mg (3 sig. figs.)