UNIT # 8 Introduction Objectives 8.18.2 8.3 Problems 1 | 2 | 3 | 4

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:

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) moles of C2H5OH moles of CO2

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

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