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Do Now
• A recipe calls for one cup of milk and three eggs per serving. You quadruple the recipe because you are expecting guests. How much milk and eggs do you need?
milk and eggs do you need?
Stoichiometry
Stoichiometry
• Stoichiometry = the proportional relationship between two or more substances during a chemical reaction.
• Reaction stoichiometry = problems involving the amount of products in relation to the amount of
amount of products in relation to the amount of reactants
– In order to correctly solve stoichiometry problems, you MUST be able to correctly balance equations!
• Stoichiometry problems are solved by using ratios from the balanced equation.
Mole Ratio
Al2O3 (l) Æ
• Balanced equations show proportions
• A balanced chemical equation is very similar to a recipe in that the coefficients show the proportions of the reactants and products
proportions of the reactants and products involved in the reaction.
– For example: 2 H2 + O2 Æ 2 H2O
So, 2 moles of hydrogen react with 1 mole of oxygen to produce 2 moles of water.
Moles to Moles
• You can use mole ratios to determine how much of a reactant is needed to produce a quantity of product, and vice versa.
• Since the coefficients in a balanced equation show the relative numbers of moles of the h
h
l
b
f
l
f h
substances in a reaction, ALWAYS convert between amounts in moles!
– For example: Balanced Equations
Al (s) + O2 (g)
• Steps to converting between amounts in moles:
1. Identify the amount in moles that you know from the problem.
2. Using coefficients from the balanced equation, set up the mole ratio with the known substance on the bottom and the unknown substance on top.
3. Multiply the original amount by the mole ratio.
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Moles to Moles Setup
Amount of known moles x unknown moles = unknown moles
known moles
Moles to Moles Example
• How many moles of hydrogen are needed to prepare 312 moles of ammonia?
N2 + 3H2 Æ 2NH3
Mole Ratio
Moles to Moles Example
• How many moles of hydrogen are needed to prepare 312 moles of ammonia?
N2 + 3H2 Æ 2NH3
• Amount of NH
f 3=
• Amount of H2 =
• From the equation: ___ mol H2 = ___ mol NH3
Mole to Mole Problems
Al2O3 (l) Æ
Al (s) + O2 (g)
• How many moles of Al(s) will be produced from the decomposition of 13.0 mol of Al2O3(l)?
Moles to Moles Example
• How many moles of hydrogen are needed to prepare 312 moles of ammonia?
N2 + 3H2 Æ 2NH3
• 312 mol NH
3 2
l
3
l 2 = ??
3 x 3 mol H
2 mol NH3
= 468 mol H2
Mole to Mole Problems
Al2O3 (l) Æ
Al (s) + O2 (g)
• How many moles of O2(g) will be produced when 36 mol of Al(s) are produced from the decomposition of Al2O3(l)?
decomposition of Al
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Mole to Mole Problems
Al2O3 (l) Æ
Al (s) + O2 (g)
• How many moles of O2(g) are produced when 15 mol of Al2O3(l) react?
Moles to Mass Stoichiometry (
(and vice versa)
)
Mass Calculations
Moles to Mass (and vice versa)
• Substances are usually measured by mass, so before using the mole ratio, you will need to convert from mass to moles!
• The conversion factor for converting between mass and moles is the molar mass of the substance.
• Molar mass = the sum of the atomic masses of the elements using the chemical formula
• To convert between Moles and Mass, you will need to find the gram‐formula mass and use your reference tables
• The formula for mole to mass calculations is The formula for mole to mass calculations is
on Table T (page 12)
• Number of moles = given mass gram‐formula mass
– Molar mass is also known as: Gram‐Formula Mass, and Molecular Mass
Mass to Mole Example
Mass to Mole Example
• N2 + 3H2 Æ 2NH3
• How many moles of H2 can be made from 22 g of H2 and excess N2?
• N2 + 3H2 Æ 2NH3
• How many moles of H2 can be made from 22 g of H2 and excess N2?
• Given:
Gi
mass of H2 = 22 g
molar mass of H2 = 2.02 g/mol
moles of NH3 = unkwown
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Mass to Mole Example
• N2 + 3H2 Æ 2NH3
• How many moles of H2 can be made from 22 g of H2 and excess N2?
Moles of H
l
f 2 = 22 g H
22
2
2 g/mol H2
11 Moles of H2
Mole to Mass Practice
• Fe2O3 + Al Æ Fe + Al2O3
• How many moles of Fe2O3 are needed to completely react with 135 g Fe2O3?
Mass to Moles Practice
Mass to Moles Practice
• Fe2O3 + Al Æ Fe + Al2O3
• How many grams of Al2O3 can form when 23.6 mole Al2O3 react with excess Fe2O3?
• Fe2O3 + Al Æ Fe + Al2O3
• How many moles of Fe2O3 react with excess Al to make 475 g Fe2O3?
Moles to Mass Practice
• Fe2O3 + Al Æ Fe + Al2O3
• How many grams of Fe will form when .976 moles Fe form?
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