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Stoichiometry
The study of quantities of substances in chemical reactions
Interpreting Chemical EquationsN2 + 3 H2 2 NH3
Particles:1 molecule of Nitrogen reacts with 3 moleculesof Hydrogen to produce 2 molecules of Ammonia (NH3).
Moles: 1 mole of Nitrogen reacts with 3 moles of Hydrogen to produce 2 moles of Ammonia (NH3).
The important thing to notice is that the coefficients determine the ratio of each reactant
to each product.
MassBalanced reactions must obey the law of conservation of mass.
Using the mole idea you can determine the mass of species in this equation:
used
1 mole N2 = 28 g3 mole H2 = 6 gproduced
2 moles NH3 = 34g
Volume@STP conditions
22.4 L of N2 reacts with 67.2 L of H2 to produce
44.8L of NH3
Notice that volume of gases is NOT conserved
22.4 + 67.2 ≠ 44.8
Interpreting Chemical EquationsN2 + 3 H2 2 NH3
Which of the following are conserved in a chemical reaction?
A. Volume of GasB. MolesC. MassD. # of Particles
C.Mass
The rearrangement of atoms during the reaction can change the # of everything else. But mass is conserved.
Mole-Mole CalculationsN2 + 3 H2 2 NH3 Relating moles of
reactants to moles of products.
We do not always use full moles of reactants, or want full moles of products.
We more often deal with partial moles.
If this reaction started with 2 moles of N2, how many moles of NH3 could be produced?
1:2 --- 2:4It’s a simple ratio
For more complicated ratios use this guide
Mole-Mole CalculationsN2 + 3 H2 2 NH3 So if we start with .75
moles of N2, how much ammonia is produced?
How many moles of H2must be used?
If we want to produce 3.18 moles of ammonia, how many moles of hydrogen do we need to use?
Known moles x
Coefficient of what you want to find out
Coefficient of what you know
4Al + 3 O2 2 Al2O3
Write three mole ratios from this equation
How many moles of Aluminum metal are needed to produce 3.7 moles of Aluminum Oxide?
How many moles of O2 are required to react completely with 14.8 moles of Al?
Calculate the moles of Al2O3 formed when 0.78n of O2 react with Al.
Mass-Mass CalculationsN2 + 3 H2 2 NH3
Our tools do not directly measure moles.
The balances measure…Mass (g)
So we often must convert to moles from mass, and mass from moles.
Solving mass-mass problems:
1. Convert the given quantity to MOLES.(Divide mass by molar mass)
2. Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
3. Convert the answer in # 2 to mass.(Multiply by molar mass)
Mass-Mass CalculationsN2 + 3 H2 2 NH3
Solving mass-mass problems:
1. Convert the given quantity to MOLES.(Divide mass by molar mass)
2. Calculate unknown moles by multiplying by a mole ratio. (unknown/known)
3. Convert the answer in # 2 to mass.(Multiply by molar mass)
If 8.5 grams of Nitrogen are reacted, what mass of ammonia can be produced?
If 16.4 grams of Hydrogen are reacted, how many grams of Nitrogen are needed to fully react?
K2O + H2O 2 KOH
1. Calculate the mass of KOH produced when 4.80 g of K2O is reacted.
2. How many grams of water are needed for this reaction?
3. If 50.0 g of KOH is produced, what is the mass of Water used?
Solving mass-mass problems:
1. Convert the given quantity to MOLES.(Divide mass by gfm)
2. Calculate unknown moles by multiplying by a mole ratio.(unknown/known)
3. Convert the answer in # 2 to mass.(Multiply by gfm)
2 Al + 3 Cl2 2 AlCl3
If 70.9 g of Chlorine Gas are used, how many moles of Aluminum Chloride can be produced?
Solving mass-mass problems:
1. Convert the given quantity to MOLES.(Divide mass by molar mass)
2. Calculate unknown moles by multiplying by a mole ratio.(unknown/known)
What is the mass of the AlCl3 ?
3. Convert the answer in # 2 to mass.(Multiply by molar mass)
Other legs of the mole wheel
If given a mass of a reactant
you can find # of particles produced
If given the volume (@STP) of a reactant
you can find the mass produced
If given the # of molecules of a product
you can find # molecules of reactant
2 H2O2 2 H2O + O2
Starting with 29.2 g of hydrogen peroxide
How many molecules of Oxygen are produced?
What is the volume of the Oxygen gas produced? (assume STP conditions)
What mass of water is produced?
MnO2
Limiting Reagent analogy
IF you have:
40 slices of turkey
28 slices of cheese
6 heads of lettuce
1 mole of tomatoes
Jars of mayo and mustard
2 slices of bread
How many sandwiches can you make?
What limits your production of sandwiches?
Bread is your “limiting reagent”
Limiting Reagent analogy
Your reaction is your recipe Seldom do you have perfect amounts of each
ingredient.
The limiting reagent (reactant) is what determines the amount that is produced.
The other ingredients are considered to be excess reagents Cheese, turkey, tomatoes…
Limiting Reagent Definition
Limiting Reactant: (a.k.a. Limiting Reagent)
The substance that controls the quantity that can form In a chemical reaction.
Excess Reactant: (a.k.a. excess reagent)
The substance that is not used up completely in a reaction.
Determining Limiting Reagent2 Na + Cl2 2 NaCl
Starting with 6.7n of Na, and 3.2n of Cl2
How many moles of NaCl can be produced?
A. Determine limiting reagent
B. Use limiting reagent to determine amount of product formed.
1. Start with a known amount of one reactant.
2. Use the mole ratio to determine the amount of product.
3. Determine how much the other reactant would produce.
4. The reactant producing the LEAST is limiting.
2 Na + Cl2 2 NaClStarting with 6.70n of Na, and 3.20n of Cl2How many moles of NaCl can be produced?
If 6.7n Na are used
6.7 x ½ = 3.35n Cl2 needed
Compare the 3.35n Cl2 needed,to 3.20n Cl2 you have.
We have less Cl2 than we need so Chlorine is the limiting reagent.
Use the limiting reagent to determine the moles of NaCl produced.
1. Start with a known amount of one reactant.
2. Use the mole ratio to determine the amount of product.
3. Determine how much the other reactant would produce.
4. The reactant producing the LEAST is limiting.
2 Na + Cl2 2 NaClStarting with 6.70n of Na, and 3.20n of Cl2How many moles of NaCl can be produced?
We have determined the 3.2n of Cl2 to be limiting.
Now it becomes a regular mole-mole problem.
3.2n Cl2 x 2/1 =
6.4n NaCl
You could then find the mass of NaCl produced.
What determines the amount of product?
The limiting reagent!
16 Cu + S8 8 Cu2S
Starting with 1.25n Cu and .78n of S
determine the moles of Copper (I) Sulfide made.
What mass of Cu2S is made?
99.4 g of Copper sulfide
0.625 n of Cu2S
MassMoles
Mg + 2 HCl MgCl2 + H2
How many grams of Hydrogen can be produced when 6.00 g of HCl is reacted with 5.00 g of Mg?
1. Convert each reactant to moles.2. Determine limiting reagent by multiplying by the
mole ratio.3. Use limiting reagent to determine moles of product.4. Multiply moles of product by GFM to find mass
Find the volume of this Hydrogen @ STP
Massmoles
2 H2 + O2 2 H2O
Given 40grams of Hydrogen and 90 grams of Oxygen, determine the limiting reagent.
Determine the mass of water that can be produced.
How much excess reagent remains?
Mass Moles
Percent Yield
Why don’t you always get 100% on your chemistry tests?
There are many possible reasons You went skiing during the important lectures.
You worked on your AP history homework instead of studying for chem.
Rippet is not as good as he thinks he is.
% Yield
In Chemical Reactions we don’t always get 100% of the products our equations predict.
Remember the Magnesium Burning Lab? 2 Mg + O2 2 MgO
Theoretically all the Mg should have converted to MgO.
In reality, some things went wrong.
% Yield
An equation predicts the THEORETICAL YIELD
When a reaction is carried out in the lab we get
ACTUAL YIELD
2 Mg + O2 2 MgO
We reacted 2.5g of Mg, what mass of MgO could be produced?
Assume there is unlimited O2 from the air so Mg is the limiting reagent.
0.103 n of Mg x 2/2 = 0.103n MgO
We only had 3.61 grams produced…So we do NOT have 100% yield!
% Yield
Actual (lab results)
Theoretical (equation prediction)X 100 %
Multiply by 100, then add the “%” sign
3.61 / 4.15 = .869
.869 x 100 +”%”
= 86.9% Yield
CaCO3 CaO + CO2
What is the % Yield if 24.8 g of CaCO3 is heated to give 13.1g of CaO?
1. Convert g CaCO3 to moles of CaCO3 .Only 1 reactant, so it’s the limiting reagent.
Mole ratio is 1:1
2. Convert moles of CaO to grams CaO.
3. Use % Yield Formula: actual x 100%
theoretical
2 Al + 3 CuSO4 Al2(SO4)3 + 3 Cu
What is the % yield when 4.65g of Copper is made when 1.87g of Al reacts with excess Copper Sulfate.
1. Excess means there is plenty, the other reagent is limiting.
2. Convert grams Al to moles Al.
3. Find moles of Cu expected (multiply by mole ratio)
4. Convert moles Cu to grams Cu.1. This is the Theoretical amount of Copper
5. Use the % Yield Formula.
Which balloon will have the greatest volume of gas?NaHCO3 + HC2H3O2 NaC2H3O2 + CO2 + H2O
Reaction Mass of NaHCO3
Moles of Acetic Acid
1 0.50g 0.02n
2 1.00g 0.02n
3 1.50g 0.02n
4 2.00g 0.02n
5 3.00g 0.02n
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