Mass-Mole and Mole-Mass
Stoichiometry
Ck12 Science
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Printed: October 4, 2015
AUTHOR
Ck12 Science
www.ck12.org
C HAPTER
Chapter 1. Mass-Mole and Mole-Mass Stoichiometry
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Mass-Mole and Mole-Mass
Stoichiometry
• Perform calculations involving conversions of mass to moles.
• Perform calculations involving conversions of moles to mass.
Need nails?
When you are doing a large construction project, you have a good idea of how many nails you will need (lots!).
When you go to the hardware store, you don’t want to sit there and count out several hundred nails. You can buy
nails by weight, so you determine how many nails are in a pound, calculate how many pounds you need, and you’re
on your way to begin building.
While the mole ratio is ever-present in all stoichiometry calculations, amounts of substances in the laboratory are
most often measured by mass. Therefore, we need to use mole-mass calculations in combination with mole ratios to
solve several different types of mass-based stoichiometry problems.
Mass to Moles Problems
In this type of problem, the mass of one substance is given, usually in grams. From this, you are to determine the
amount in moles of another substance that will either react with or be produced from the given substance.
mass of given → moles of given → moles 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.
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Sample Problem: Mass-Mole Stoichiometry
Tin metal reacts with hydrogen fluoride to produce tin(II) fluoride and hydrogen gas according to the following
balanced equation.
Sn(s) + 2HF(g) → SnF2 (s) + H2 (g)
How many moles of hydrogen fluoride are required to react completely with 75.0 g of tin?
Step 1: List the known quantities and plan the problem.
Known
• given: 75.0 g Sn
• molar mass of Sn = 118.69 g/mol
• 1 mol Sn = 2 mol HF (mole ratio)
Unknown
• mol HF
Use the molar mass of Sn to convert the grams of Sn to moles. Then use the mole ratio to convert from mol Sn to
mol HF. This will be done in a single two-step calculation.
g Sn → mol Sn → mol HF
Step 2: Solve.
75.0 g Sn ×
2 mol HF
1 mol Sn
×
= 1.26 mol HF
118.69 g Sn 1 mol Sn
Step 3: Think about your result.
The mass of tin is less than one mole, but the 1:2 ratio means that more than one mole of HF is required for the
reaction. The answer has three significant figures because the given mass has three significant figures.
Moles to Mass Problems
In this type of problem, the amount of one substance is given in moles. From this, you are to determine the mass of
another substance that will either react with or be produced from the given substance.
moles of given → moles of unknown → mass of unknown
The moles of the given substance are first converted into moles of the unknown by using the mole ratio from the
balanced chemical equation. Then, the moles of the unknown are converted into mass in grams by use of the molar
mass of that substance from the periodic table.
Sample Problem: Mole-Mass Stoichiometry
Hydrogen sulfide gas burns in oxygen to produce sulfur dioxide and water vapor.
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Chapter 1. Mass-Mole and Mole-Mass Stoichiometry
2H2 S(g) + 3O2 (g) → 2SO2 (g) + 2H2 O(g)
What mass of oxygen gas is consumed in a reaction that produces 4.60 mol SO2 ?
Step 1: List the known quantities and plan the problem.
Known
• given: 4.60 mol SO2
• 2 mol SO2 = 3 mol O2 (mole ratio)
• molar mass of O2 = 32.00 g/mol
Unknown
• mass O2 = ? g
Use the mole ratio to convert from mol SO2 to mol O2 . Then convert mol O2 to grams. This will be done in a single
two-step calculation.
mol SO2 → mol O2 → g O2
Step 2: Solve.
4.60 mol SO2 ×
3 mol O2
32.00 g O2
×
= 221 g O2
2 mol SO2
1 mol O2
Step 3: Think about your result.
According to the mole ratio, 6.90 mol O2 is produced with a mass of 221 g. The answer has three significant figures
because the given number of moles has three significant figures.
Summary
• Calculations involving conversions of mass to moles and moles to mass are described.
Practice
Work problems 11-20 at the link below:
http://myweb.astate.edu/mdraganj/Moles1.html
Review
Questions
1. In the first problem, what would happen if you multiply grams Sn by 118.69 grams/mole Sn?
2. Why is a balanced equation needed?
3. Does the physical form of the material matter for these calculations?
• mass-mole calculations: mass of given → moles of given → moles of unknown
• mole-mass calculations: moles of given → moles of unknown → mass of unknown
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References
1. Image copyright phloen, 2014. http://www.shutterstock.com .
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