Chemistry Homework! Combustion!

Question

1. How many moles of carbon dioxide is produced by the complete combustion of 5 g of propane
2. Convert this answer into grams

Answer 1

Hi Lena:

For Question 1

First, write the balanced stoichiometric equation for the combustion. 

For complete combustion of hydrocarbons such as propane (C₃H₈), oxygen (O₂) reacts with the hydrocarbon to produce carbon dioxide (CO₂) and water (H₂O).

C₃H₈ + O₂  → CO₂ + H₂O  (unbalanced)

  

The balanced equation is

C₃H₈ + 5O₂  → 3CO₂ + 4H₂O

From the stoichiometric coefficients of this equation, we see that 1 mole of propane (C₃H₈) yields 3 moles of carbon dioxide. That is the mole ratio of

 C₃H₈: CO₂  is 1:3

For the remainder of this question, you will need the molar masses of C, H and O.

From the Periodic Table of the elements, these are:

C - 12.01g

H -  1.008 g

O -  16.00 g

Now, convert the 5g of propane to moles. For this, we need its molar mass.

Molar mass of C₃H₈ = 3(12.01g) + 8(1.008g) = 44.09 g

5 g C₃H₈ x 1mole C₃H₈    =  0.1134 moles C₃H₈

                 44.09 g C₃H₈

Now use the mole ratio of C₃H₈ : CO₂ to determine the number of moles of CO₂ produced from the 0.1134 moles C₃H₈.

Moles of CO₂ produced  = 0.1134 moles C₃H₈  x  3 moles CO₂     = 0.340 moles CO₂

                                                                               1  moles C₃H₈  

Answer = 0.340 moles CO₂

Alternatively, you can combine the two steps above as follows:

5 g C₃H₈   x   1 mole C₃H₈       x    3 moles CO₂     = 0.340 moles CO₂

                     44.01 g C₃H₈         1  moles C₃H₈  

 

Question 2

To find the number of grams of CO₂ produced, convert 0.340 moles CO₂ to grams

Molar mass of CO₂ = 12.01g + 2 (16.00 g) = 44.01 g

0.340 moles CO₂   x  44.01 g             =  14.97 g CO₂

                                  1 mole CO₂

Answer = 14.97 g CO₂