CHEMICAL REACTIONS

8.5 - Calculations based on Chemical Equations

8.5.2 - Mass-mass Calculations

In unit 7, we learned conversion between mass and moles (section 7.5.1) for a given compound. We will now combine this mass-to-mole conversion in stoichiometric 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 mass of the other reactant (oxygen gas).

How many grams of oxgen gas is required to burn 20.5 grams of ethyl alcohol, C₂H₅OH?

We use the basic steps for the example from section 8.5.1, and add extra steps for the conversions from moles to mass:
(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 ethyl alcohol and oxygen.
Step 4: Calculate the number of moles of oxygen by making the ratio specifically for the number of moles of C₂H₅OH from step 2a.
Step 5: Convert the number of moles of oxygen obtained in step 4 to mass of oxygen.

Let's analyze:

1. Steps 1, and 2 are the same as the example in section 8.5.1.
2. Step 2a is necessary because the mass of C₂H₅OH must be converted to moles since the coefficients in the balanced chemical reaction give the mole ratio of the reactants. To set up stoichiometric ratios using masses would be WRONG.
3. In Step 4, we calculate the number of moles of oxygen 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.
4. After we obtain the number of moles of oxygen from Step 4, we need to convert it to mass of oxygen in order to answer the question properly.

Summary:

Refer to section 8.5.1 for a detailed look at Step 1, 2, and 3. Click here for a detailed look at my work for Steps 2a, 3 and 5. 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 oxygen

step 5

mass of oxygen

(Note: We use the calculator answer in our calculations up until the final answer. Then we follow significant figure rules to remove the excess digits.)

Section 10.4
The Mass-Mass Stoichiometry Problems ..p251

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