CHEM 10
Brakke
ECA – Topic 06
T06D03 – Mole Concept – Stoichiometric Mole Conversions
Name…………………………………………………………
The Law of Conservation of Mass states that matter (anything that has mass and takes up space) can neither be created nor
destroyed. This concept applies to the balancing of chemical equations. In a chemical reaction, atoms must be equal on each side of
the equation or the law is not upheld. Equations are balanced by the addition of whole number coefficients in front of compounds in
a chemical reaction.
When determining how much material might be produced or needed, the mole ratio must be utilized. The mole ratio is the
relationship between moles of one compound in a chemical equation and the moles of another. For example,
KClO3  O2 + KCl (not balanced)
When potassium chlorate (KClO3) is decomposed under heat, the reaction will produce oxygen (O2) and potassium chloride (KCl). In
the balanced chemical equation below, you can see the mole ratio of one compound to another. The stoichiometric coefficient
(numbers in front of each formula used to balance the equation) can be used to find the molar ratio of one material to another.
2 KClO3  3 O2 + 2 KCl (balanced)
The mole ratio for each is:
• 2mol KClO3 = 3 mol O2
• 2 mol KclO3 = 2 mol KCl
• 3 mol O2 = 2 mol KCl
If you were asked to find the mass (in g) of KCl produced when 200g of KclO3 are decomposed, you may do so as:
200𝑔 𝐾𝐶𝑙𝑂3 𝑥
2 𝑚𝑜𝑙 𝐾𝐶𝑙
74.55 𝑔 𝐾𝐶𝑙
1 𝑚𝑜𝑙 𝐾𝐶𝑙𝑂3
𝑥
𝑥
= 121.66 𝑔 𝐾𝐶𝑙
1 𝑚𝑜𝑙 𝐾𝐶𝑙
122.55 𝑔 𝐾𝐶𝑙𝑂3 2 𝑚𝑜𝑙 𝐾𝐶𝑙𝑂3
Practice Examples:
1.
If 5.0g of aluminum metal react with a stoichiometrically equivalent (or greater) amount of chlorine gas, how much of the
salt will be produced (in grams)?
__Al (s) + __Cl2 (g)  __AlCl3 (s)
2.
If 1.0 g of dinitrogen monoxide (N2O) are decomposed, how many atoms of nitrogen AND atoms of oxygen are formed?
__N2O (g)  __N2 (g) + __O2 (g)
3.
If 4.0g of boric acid (H3BO3) are decomposed, boron oxide and water are formed. To separate the two, the mixture was
distilled to remove water. What mass of boron oxide remains?
__H3BO3 (s)  __ B2O3 (s) + __H2O (l)
4.
If 10.0 dm of benzene (C6H6(l) – density 0.877 g cm ) is combusted in air, what volume of carbon dioxide gas would be
o
3
3
3
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produced at STP (25 C, 1atm)? (FYI, 1dm =1000cm , 1dm =1L, 1cm =1mL, 1L=100mL)
__C6H6 (l) + __O2 (g)  __H2O (g) + __CO2 (g)
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CHEM 10
Brakke
ECA – Topic 06
Stoichiometric Decomposition of Hydrogen Perioxide
A very common chemical reaction is the catalyzed decomposition of hydrogen peroxide (H2O2). This reaction will be seen several
times throughout IB Chemistry as it allow us to demonstrate concepts of quantitative analysis, error, energetics, kinetics, and so on.
Today our focus is on the quantitative production of a gas. You will be asked to calculate the amount of oxygen gas that should be
produced (theoretical) and then you will complete the experiment for your actual value (experimental).
In this lab, the catalyst used (KI) will be in excess and will remain unreacted. Soap will be used to capture the oxygen gas and the
reaction will be completed in a large graduated cylinder so as to measure the production of a gas.
You are provided with about 10mL of 30% (by mass) H2O2, if the material is completely reacted into its products at STP, calculate the
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total production of oxygen gas (O2 in cm ). Be sure to assign an uncertainty to your value.
__H2O2(aq)  __H2O(l) + __O2(g)
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Density of 30% H2O2 = 1.110 g cm
Complete the lab experiment, draw a very simple diagram of the apparatus (experiment setup):
What volume of gas was produced? (assign an uncertainty value)
What is the percent error?
% 𝐸𝑟𝑟𝑜𝑟 = �
(𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 − 𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙)
� 𝑥 100
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
What is the percent yield?
% 𝑌𝑖𝑒𝑙𝑑 =
𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙
𝑥 100
𝑇ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙
Discuss the uncertainty within the experiment
• Is the percent error within the uncertainty? If not, what does that suggest?
•
Would it be possible for the percent yield to be above 100%?
•
If the percent yield is not 100%, suggest a couple reasons why this may be the case – be sure to justify.