 # Stoichiometry Chapter 3. Atomic Mass Atoms are so small, it is difficult to weigh in grams (Use atomic mass units) Atomic mass is a weighted average of.

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Stoichiometry Chapter 3

Atomic Mass Atoms are so small, it is difficult to weigh in grams (Use atomic mass units) Atomic mass is a weighted average of all masses for each isotope of an element Are not whole numbers because they are averages These are the numbers found on the periodic table for each element Example: page 80 Atoms are so small, it is difficult to weigh in grams (Use atomic mass units) Atomic mass is a weighted average of all masses for each isotope of an element Are not whole numbers because they are averages These are the numbers found on the periodic table for each element Example: page 80

The Mole Number used to count atoms It is 6.022 X 10 23 anything is one mole Found through the number of atoms in 12 grams of carbon-12 Allows calculations/predictions for reactions to be made Examples page 82, 83, 84 Number used to count atoms It is 6.022 X 10 23 anything is one mole Found through the number of atoms in 12 grams of carbon-12 Allows calculations/predictions for reactions to be made Examples page 82, 83, 84

Molar Mass (Molecular weight) Mass (grams) of one mole of a substance For a compound, add all the molar masses of the elements that make it up Examples Mg 3 P 2 Al 2 (Cr 2 O 7 ) 3 Ca(NO 3 ) 3 Mass (grams) of one mole of a substance For a compound, add all the molar masses of the elements that make it up Examples Mg 3 P 2 Al 2 (Cr 2 O 7 ) 3 Ca(NO 3 ) 3

Percent Composition Provides the contribution to a compound based on percentage To find Have the mass of each element Add to total the mass of the compound Divide the mass of the element by the mass of the compound Multiply by 100 (for a percentage) Provides the contribution to a compound based on percentage To find Have the mass of each element Add to total the mass of the compound Divide the mass of the element by the mass of the compound Multiply by 100 (for a percentage)

Percent Composition Examples: Find the percent composition of each element in : Mg 3 P 2, Al 2 (Cr 2 O 7 ) 3, and Ca(NO 3 ) 3 Mg: 54.1%P: 46.1% Al: 7.7%Cr: 44.5%O: 47.8% Ca: 17.8%N: 18.7%O: 63.6% Examples: Find the percent composition of each element in : Mg 3 P 2, Al 2 (Cr 2 O 7 ) 3, and Ca(NO 3 ) 3 Mg: 54.1%P: 46.1% Al: 7.7%Cr: 44.5%O: 47.8% Ca: 17.8%N: 18.7%O: 63.6%

Empirical formulas The lowest ratio of atoms in a molecule Can use percent compositions to determine empirical formula Based on mole ratios Examples CH for C 2 H 2 and C 6 H 6 CH 3 for C 2 H 6 The lowest ratio of atoms in a molecule Can use percent compositions to determine empirical formula Based on mole ratios Examples CH for C 2 H 2 and C 6 H 6 CH 3 for C 2 H 6

Molecular Formulas Molecular formula represents the number of each atom in a molecule Need molar mass to convert empirical formula to molecular formula Uses a ratio of molar mass of empirical formula to known molar mass Rules for determining molecular formulas 1. Use mass percentages and molar mass to determine number of moles Molecular formula represents the number of each atom in a molecule Need molar mass to convert empirical formula to molecular formula Uses a ratio of molar mass of empirical formula to known molar mass Rules for determining molecular formulas 1. Use mass percentages and molar mass to determine number of moles

Molecular Formulas 2. Determine moles of element in one mole of compound 3. The integers from step 2 are the subscripts for the elements Example Caffeine contains 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen and has a molar mass of 194.2 g/mol. What is the molecular formula for caffeine? 2. Determine moles of element in one mole of compound 3. The integers from step 2 are the subscripts for the elements Example Caffeine contains 49.48% carbon, 5.15% hydrogen, 28.87% nitrogen, and 16.49% oxygen and has a molar mass of 194.2 g/mol. What is the molecular formula for caffeine?

Caffeine Answer C 8 H 10 N 4 O 2 http://en.wikipedia.org/wiki/File:Caffeine.sv g C 8 H 10 N 4 O 2 http://en.wikipedia.org/wiki/File:Caffeine.sv g

Chemical Equations Represents a reorganization of atoms in one or more substances (reactions) Reactant(s) --------> Product(s) Atoms must be equal on both sides of the equation (balancing equations) Cannot change formulas to balance equations Occur as atoms, molecules, particles - not as grams! Represents a reorganization of atoms in one or more substances (reactions) Reactant(s) --------> Product(s) Atoms must be equal on both sides of the equation (balancing equations) Cannot change formulas to balance equations Occur as atoms, molecules, particles - not as grams!

Equation Practice Ca(OH) 2 + H 3 PO 4 -------> H 2 O + Ca 3 (PO 4 ) 2 Cr + S 8 ------> Cr 2 S 3 KClO 3 (s) -------->Cl 2 (g) + O 2 (g) Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas. Fe 2 O 3 (s) + Al(s) --------> Fe(s) + Al 2 O 3 (s) Ca(OH) 2 + H 3 PO 4 -------> H 2 O + Ca 3 (PO 4 ) 2 Cr + S 8 ------> Cr 2 S 3 KClO 3 (s) -------->Cl 2 (g) + O 2 (g) Solid iron(III) sulfide reacts with gaseous hydrogen chloride to form solid iron(III) chloride and hydrogen sulfide gas. Fe 2 O 3 (s) + Al(s) --------> Fe(s) + Al 2 O 3 (s)

Abbreviations (s) solid, (l) liquid, (g) gas (aq) aqueous heat  catalyst

Stoichiometry Method of calculating amounts of reactants needed or products produced Uses mole ratios (moles to moles) and molar mass (grams/mole) to perform calculations First step is always to balance the equation!! How much carbon dioxide is absorbed by 1 kg of lithium hydroxide LiOH (s) + CO 2 (g) -----> Li 2 CO 3 (s) + H 2 O(l) Answer is 920 grams (page 106) Method of calculating amounts of reactants needed or products produced Uses mole ratios (moles to moles) and molar mass (grams/mole) to perform calculations First step is always to balance the equation!! How much carbon dioxide is absorbed by 1 kg of lithium hydroxide LiOH (s) + CO 2 (g) -----> Li 2 CO 3 (s) + H 2 O(l) Answer is 920 grams (page 106)

Limiting Reactants (Reagents) The reactant controlling the amount of product made in a reaction Can use stoichiometric calculations to determine the reactant limiting the products Used to determine the theoretical yield Example: N 2 + H 2 ------> NH 3 What mass of ammonia can be produced from 100 grams of N and 500 grams of H? What is the limiting reactant? How much unreacted materials remain? The reactant controlling the amount of product made in a reaction Can use stoichiometric calculations to determine the reactant limiting the products Used to determine the theoretical yield Example: N 2 + H 2 ------> NH 3 What mass of ammonia can be produced from 100 grams of N and 500 grams of H? What is the limiting reactant? How much unreacted materials remain?

Percent Yield Comparison of the actual amount produced to the theoretical yield Recorded as a percentage Percent yield = actual yield X 100% theoretical yield Example: Aluminum burns in bromine producing aluminum bromide. In a lab, 6.0 g of aluminum react with excess bromine. 50.3 g of aluminum bromide are produced. What are the values of the three types of yield? Comparison of the actual amount produced to the theoretical yield Recorded as a percentage Percent yield = actual yield X 100% theoretical yield Example: Aluminum burns in bromine producing aluminum bromide. In a lab, 6.0 g of aluminum react with excess bromine. 50.3 g of aluminum bromide are produced. What are the values of the three types of yield?

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