CHEMICAL REACTIONS

8.5 - Calculations based on Chemical Equations

8.5.3 - Mass-mole Calculations

In section 8.5.2 we took a detailed look at how to carry out Mass-mass type calculations.

Using the same example from section 8.5.1, we now pose another question. This time you will be given the mass of one reactant (C₂H₅OH) and you will be asked to find the moles of the product (carbon dioxide gas).

How many moles of carbon dioxide gas is produced when 20.5 grams of ethyl alcohol, C₂H₅OH, is burned?

The steps involved to answer this question is modified from the example in section 8.5.2:
(Highlighted in red are the modifications needed in order to answer this question.)

Step 1: Identify the chemical equation involved: a combustion reaction.
Step 2: Write the balanced chemical equation (review balancing equations, section 8.3).
Step 2a: Convert the mass of C₂H₅OH to moles of C₂H₅OH.
Step 3: Determine the stoichiometric ratio of the substances that you are working with, specifically between C₂H₅OH and CO₂.
Step 4: Calculate the number of moles of CO₂ by making the ratio specifically for the number of moles of C₂H₅OH from step 2a.

Let's analyze:

1. Step 1, 2 and 2a are the same as in sections 8.5.1 and 8.5.2.
2. We modify Step 3 because the question is interested in the amount of carbon dioxide produced from the burning of C₂H₅OH. So we need to determine the stoichiometric ratio of CO₂ : C₂H₅OH from the balanced equation.
3. In Step 4, we calculate the number of moles of CO₂ that is required to burn the number of moles of C₂H₅OH. The number of moles of C₂H₅OH available is determined from the given mass of 20.5 grams C₂H₅OH in Step 2a.

Summary:

Refer to section 8.5.2 for a detailed look at Step 1, 2, and 2a. Click here for a detailed look at my work for Steps 3 and 4. If you need to see the specific steps, click on the links below.

Mass of C2H5OH (20.5 grams)

step 2a

moles of C2H5OH

step 4

moles of CO2

Section 10.4
The Mass-Moles and Moles to Mass Examples ..p253

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