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
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:
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
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.
- Steps 1, and 2 are the same as the example in section 8.5.1.
- 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.
- 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.
- 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.
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
moles of C2H5OH
moles of oxygen
Mass-Mass Stoichiometry Problems ..p251