Mass-Mass Stoichiometry
Ck12 Science
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Printed: May 13, 2015
AUTHOR
Ck12 Science
www.ck12.org
Chapter 1. Mass-Mass Stoichiometry
C HAPTER
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Mass-Mass Stoichiometry
• Perform calculations involving the determination of the mass of product based on the given mass of the
reactant.
How many walnuts are needed to equal 250 grams?
I want to send 250 grams of shelled walnuts to a friend (don’t ask why –just go with the question). How many
walnuts in shells do I need to buy? To figure this out, I need to know how much the shell of a walnut weighs (about
40% of the total weight of the unshelled walnut). I can then calculate the mass of walnuts that will give me 250
grams of shelled walnuts and then determine how many walnuts I need to buy.
Mass to Mass Problems
Mass-mass calculations are the most practical of all mass-based stoichiometry problems. Moles cannot be measured
directly, while the mass of any substance can generally be easily measured in the lab. This type of problem is three
steps and is a combination of the two previous types.
mass of given → moles of given → moles of unknown → mass of unknown
The mass of the given substance is converted into moles by use of the molar mass of that substance from the periodic
table. Then, the moles of the given substance are converted into moles of the unknown by using the mole ratio from
the balanced chemical equation. Finally, the moles of the unknown are converted to mass by use of its molar mass.
Sample Problem: Mass-Mass Stoichiometry
Ammonium nitrate decomposes to dinitrogen monoxide and water according to the following equation.
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NH4 NO3 (s) → N2 O(g) + 2H2 O(l)
In a certain experiment, 45.7 g of ammonium nitrate is decomposed. Find the mass of each of the products formed.
Step 1: List the known quantities and plan the problem.
Known
•
•
•
•
•
given: 45.7 g NH4 NO3
1 mol NH4 NO3 = 1 mol N2 O = 2 mol H2 O (mole ratios)
molar mass of NH4 NO3 = 80.06 g/mol
molar mass of N2 O = 44.02 g/mol
molar mass of H2 O = 18.02 g/mol
Unknown
• mass N2 O = ? g
• mass H2 O = ? g
Perform two separate three-step mass-mass calculations as shown below.
g NH4 NO3 → mol NH4 NO3 → mol N2 O → g N2 O
g NH4 NO3 → mol NH4 NO3 → mol H2 O → g H2 O
Step 2: Solve.
1 mol NH4 NO3
1 mol N2 O
44.02 g N2 O
×
×
= 25.1 g N2 O
80.06 g NH4 NO3 1 mol NH4 NO3
1 mol N2 O
1 mol NH4 NO3
2 mol H2 O
18.02 g H2 O
45.7 g NH4 NO3 ×
×
×
= 20.6 g N2 O
80.06 g NH4 NO3 1 mol NH4 NO3
1 mol H2 O
45.7 g NH4 NO3 ×
Step 3: Think about your result.
The total mass of the two products is equal to the mass of ammonium nitrate which decomposed, demonstrating the
law of conservation of mass. Each answer has three significant figures.
Summary
• Mass-mass calculations involve converting the mass of a reactant to moles of reactant, then using mole ratios
to determine moles of product which can then be converted to mass of product.
Practice
Read the material at the link below, then do the mass-mass problems at the link found at the bottom of the page:
http://www.chemteam.info/Stoichiometry/Mass-Mass.html
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Chapter 1. Mass-Mass Stoichiometry
Review
Questions
1. If matter is neither created nor destroyed, why can’t we just go directly from grams of reactant to grams of
product?
2. Why is it important to get the subscripts correct in the formulas?
3. Why do the coefficients need to be correct?
• mass-mass calculations: mass of given → moles of given → moles of unknown → mass of unknown
References
1. Pauline Mak. http://www.flickr.com/photos/__my__photos/5591677002/ .
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