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Stoichiometry: the mass relationships between reactants and products. We will use the molar masses ( amount of
grams in one mole of a element or compound) from the periodic table
And the coefficients from the balanced equations to solve problems
When the following reaction occurs:Ba + Cl2 BaCl2
One atom of barium reacts with 1 molecule of chlorine to make one molecule of barium chloride.
It does not mean that 1 gram of barium reacts with 1 gram of chlorine to make 1 gram of barium chloride!
Remember- barium and chlorine are different sizes and have different masses!
We use stoichiometry to relate the atoms to masses- something we can easily measure.
Not only signify the ratio of atoms that react- also the ratio of moles of that substance involved.
Mole Ratio: a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction.
For example:
2Al2O3 4Al + 3O2
The mole ratio of Al2O3 to Al is 2:4, the mole ratio of Al2O3 to O2 is 2:3
You can use the mole ratio to convert from moles of one substance to moles of another.
Use the following reaction to answer the question: 4NH3 + 6NO 5N2 + 6H2O
1.If I have 20 moles of NH3, how many moles of N2 can I make?
2o mols NH3 5 mols N2 = 25 mols N2
4 mols NH3
Moles given
Coefficient wanted
Coefficient given
Moles wanted
If I want to make 2 moles of NaCl, How many mols of NaI do I need?2 mols
If I have 10 moles of Cl2, how many moles of I2 can I make?10 mols
If I want 15 mols of I2, how many mols of NaI do I need?30 mols
You can use molar mass of the substance to convert from moles to grams.
Remember, the molar mass on the periodic table is the number of grams in 1 mole.
How many grams in 1 mole of Na? 23 g
How many grams in 1 mole of NaCl?58.5 g
1 mol given
Molar mass of given
Grams of given
Coefficient of given
Coefficient of wanted Moles of
wanted
If I have 100 g of NaI, how many moles of I2 can I make? (molar mass of NaI is 150 g/mol)
100 g NaI 150 g
NaI
1 mol NaI
2 mol NaI
1 mol I20.033 mol I2
If I want to make 150 g of NaCl, how many moles of NaI do I need?
2.6 mols
Moles of given
1 mol wanted
Molar mass of wanted
Coefficient of wanted grams of
wantedCoefficient of given
If I have 10 moles of NaI, how many grams of I2 can I make?
10 moles NaI
2 moles NaI
1 mol I2
1 mol I2
254 g I2 = 1270 g
I2
If I need 15 moles of NaCl, how many grams of Cl2 do I need?
532 grams
grams of given
Molar mass of given
1 Mol given
Coefficient of given
Coefficient of wanted
Molar mass of wanted1 mole of wanted
= Grams of wanted
If I have 500 g of NaI, how many grams of I2 can I make?
2 mol NaI
1 mol I2
254 g I2= 423.3 g I2
1 mol I21 mol NaI150g
NaI
500 g NaI
If I want 150 g of NaCl, how many grams of I2 will I make?
325.6 g I2
Remember 1 mol = 22.4 L of a gas.
Are you LOST???
1. When you do a chemical experiment one of the reactants is going to be used up first.
2. The reactant that is used up first in a chemical experiment is called the limiting reactant
3. You can’t tell limiting reactants until you look at mole ratios
What is the limiting reactant in the picture?
4. How to find the limiting reactantA. Step 1: Balance equationB. Step 2: Convert both reactants to molesC. Step 3: Start with one reactant and find how
many grams of a product you will get (choose any product)
D. Step 4: Do the same with the other reactantE. Step 5: whichever gives less product is the
limiting reactant
1. Use the following equation to answer the next question
____ MnO2 + ____HCl ____MnCl2 + ____H2O + ____ Cl2
You have 5.47 grams of MnO2 and 9.861 grams of HCl. What is the limiting reactant?
A. Step 1: balance the chemical equation
____ MnO2 + ____HCl ____MnCl2 + ____H2O + ____ Cl21 4 1 2 2
B. Step 2: using the reactants, find the grams of a product stoichiometry. Essentially, you will be doing two stoichiometry problems. In this example I chose MnCl2 as the product to find grams of
5.47 g of MnO2
1 mol of MnO2
87 g of MnO2 1 mol of MnO2
1 mol of MnCl2 1 mol of MnCl2
125.84 g of MnCl2
7.91 g of MnCl2
9.861 g of HCl 1 mol of HCl
36.45 g of HCl
4 mol of HCl
1 mol of MnCl2 1 mol of MnCl2
125.84 g of MnCl2
8.511 g of MnCl2
C. Step 3: Compare the two numbers you found for your products. The lower of the two numbers is the limiting reactant. In other words, it is the reactant that will run out first.
MnO2 is the limiting reactant
1. Once the limiting reactant is found, we can use that value as the theoretical yield
What this tells us is that we should get this amount of product after we do the experiment.
2. The amount of product that actually forms during an experiment is called actual yield
It’s impossible to get 100% yields from our in class experiments.
1. To find out how close to the maximum yield you have you calculate a percent yield
2. Percent error tells you how far off you are from producing the maximum yield.
3. Percent yield and percent error should add up to 100%
Percent yield equation
Percent error equation
100 Yield lTheoretica
Yield Actualx
100 Yield lTheoretica
Yield Actual- lTheoreticax
Use the equation for the following problems
____Al + ____O2 ____Al2O3
1. What is the theoretical yield of Al2O3 if you start with 5.782 grams of Al?
A. Step 1: balance the chemical equation
____Al + ____O2 ____Al2O34 23
B. Convert 5.782 grams of Al to grams of Al2O3
using stoichiometry. Doing this will give you the theoretical yield. In actuality, you have been finding theoretical yields the whole unit.
5.782 g of Al 1 mol of Al
26.98 g of Al 4 mol of Al
2 mol of Al2O3 1 mol of Al
101.96 g of Al2O3
10.93 g of Al2O3
2. If the actual yield is 8.64 grams of Al2O3,what is the percent yield?
100 g 10.82
64.8x
g
= 79.9%
3. What is the percent error?
100 g 10.93
64.893.10x
gg
= 21%