Stoichiometry practice

Question

How many moles of carbon dioxide are produced when 10.0 moles of propane, C

3 H

8 , are

burned in excess oxygen gas in a gas grill?

C

3 H

8 + O

2 → _____ + _____

How many moles of carbon dioxide are produced when 10.0 moles of propane, C3 H8 , are burned in excess oxygen gas in a gas grill? C3 H8 + O2 → _____ + _____ many moles of carbon dioxide are produced when 10.0 moles of propane, C

3 H

8 , are

burned in excess oxygen gas in a gas grill?

C

3 H

8 + O

2 → _____ + _____

Answer 1

Step 1: Determine the products being produced. Since this is a combustion reaction, key word "burned with excess oxygen gas O₂". The products of a combustion reaction are always CO₂ (carbon dioxide) and H₂O (water).

C3 H8 + O2 → _____ + _____

C₃H₈ + O₂ → CO₂ + H₂O

Step 2: Balance the equation by listing all the elements on the reactants and products side

Reactions: 3C 8H + 2O

Product: 1C 2O + 2H 1O

Balance the elements in the order of carbon, hydrogen, then oxygen

There are 3C on the reactions, but 1C on the product side. Let's multiple by 3 to balance carbons.

C₃H₈ + O₂ → 3CO₂ + H₂O

The change would be 3 x (1C 2O) = 3C 6O

Reactions: 3C 8H + 2O

Product: 3C 6O + 2H 1O

Now balance the hydrogen atoms

There are 8 H atoms on the reactants side, but 2 on the product side. Let's multiple by 4 to balance the hydrogens.

C₃H₈ + O₂ → 3CO₂ + 4H₂O

The change would be 4 x (2H O) = 8H 4O

Reactions: 3C 8H + 2O

Product: 3C 6O + 8H 4O

Now let's balance the oxygen atoms.

The reactants side has 2 oxygen atoms and the product side has 10 oxygen atoms. We need to multiple by 5 on the reactants side to balance.

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

The change would be 5 x (2O) = 10O

Reactions: 3C 8H + 10O

Product: 3C 6O + 8H 4O = 3C 8H 10O

Our equation is balanced with the same number atoms of each element (3 carbon, 8 hydrogen, and 10 oxygen) on the reactants and product side.

Step 3: Determine how many moles of CO₂ are produced with 10 moles of propane C₃H₈

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Set up your equation to solve for x = moles of carbon dioxide

Start with what you know you have, which is 10 moles of C₃H₈

Setup the equation based on the ratio of reactant to product.

Based on the equation 1 mole of propane produces 3 moles of carbon dioxide

10 mole C₃H₈ 3 mol CO₂

------------------ X ------------------ = 30 mol CO₂

1 mol C₃H₈

Why do you place carbon dioxide on top in the equation? When you divide by the same units, you will cancel them out. When you multiply by the same units, they are not canceled out. Use your knowledge of what the question is asking to set up your equations and get to the correct final unit.

10 mol C₃H₈ (3 mol CO₂)

---------------------------------- = 30 mol CO₂

1 mol C₃H₈

Wrong equation set up below, because the units do not cancel out.

10 mole C₃H₈ 1 mol C₃H₈ 10 mol (C₃H₈)²

------------------ X ------------------ = ---------------------------

3 mol CO₂ 3 mol CO₂

Tip: Always write out the units and elements in all equations.