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# 3.3 Atomic and Molecular Weights

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3.3 Atomic and Molecular Weights. Demo: large bag of Styrofoam peanuts, bottle of sand, 1 L beaker, 18 mL of water How can we count very large numbers of particles? Demo: weigh pennies of various ages. Masses of Coins. Masses of Coins. - PowerPoint PPT Presentation

### Text of 3.3 Atomic and Molecular Weights

Chapter 4: Chemical Equations and Quantitative Relationships3.3 Atomic and Molecular Weights
Demo: large bag of Styrofoam peanuts, bottle of sand, 1 L beaker, 18 mL of water
How can we count very large numbers of particles?
Demo: weigh pennies of various ages
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Masses of Coins
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Atomic and Molecular Weights
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Atomic and Molecular Weights
Why do we use a C-12 standard for the mass of atoms of elements?
How can we determine these masses?
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How do we get average atomic weights?
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Atomic and Molecular Weights
atomic weight: average mass of 1 atom of an element, expressed in amu
formula weight: sum of the atomic weights of each atom in a chemical formula
What is the formula weight of CaCl2?
molecular weight: same as formula weight when the chemical formula is a molecular formula
What is the molecular weight of H2CO3?
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Percentage Composition from Formulas
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What do these substances all have in common?
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Hg
CuSO4.5H2O
NaCl
Sn
Fe2O3
S8
Fe
Cu
H2O
Molar Mass and Moles
Describe the difference between molar mass, molecular weight, and atomic weight.
How do we get these quantities?
Calculations:
How do we carry out these conversions?
mass moles: Use molar mass
moles number of particles: Use NA
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Mass of writing = 5.473 g - 5.448 g = 0.025 g
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0.025 g x 1 mol/100.08 g = 0.00025 mol CaCO3
0.00025 mol CaCO3 contains 0.00025 mol Ca, 0.00025 mol C, 0.00075 mol O
0.00025 mol Ca x 40.08 g/mol = 0.0100 g Ca
0.00025 mol C x 12.01 g/mol = 0.00300 g C
0.00075 mol O x 16.00 g/mol = 0.0120 g O
sum = 0.0100 + 0.00300 + 0.0120 = 0.0250 g
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50 g O x 1 mol/16.0 g = 3.125 mol O
50 g S x 1mol/32.066 g = 1.559 mol S
mol O/mol S = 3.125/1.559 = 2.004 or 2
SO2
60 g O, 40 g S 3.75 mol O, 1.25 mol S
SO3
Molecular formula from empirical formula
How to calculate a molecular formula if a molar mass is known?
Empirical formula = CH2O, MM = 90 g/mol
What is the molecular formula?
Empirical formula mass = 12 + 2 + 16 = 30
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Group Quiz Format
Place TA name and section letters (BA, or BB, or … , or BH) in upper left corner of paper.
Place name of group members participating in the quiz in the upper right corner. Please use correct spelling and write or print legibly.
Show work for partial credit.
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Group Quiz 4
A compound contains 71.0% potassium and 29.0 % oxygen. The molar mass of the compound is about 110 g/mol.
What is the empirical formula of the compound?
What is the molecular formula of the compound?
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Answers to group quiz
Assume a sample of 100 g, which then contains 71.0 g K and 29.0 g O.
71.0 g K x 1 mol/39.1 g = 1.82 mol K
29.0 g O x 1 mol/16.0 g = 1.81 mol O
ratio = 1.82 mol K/1.81 mol O = 1.00
empirical formula = KO
formula = (KO)2 = K2O2
Mass-Mole Conversions
MM = 100 g/mol
How many moles in 20 g?
How many moles in 25 g of NaOH?
MM = 40 g/mol
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CaCl2 + Na2CO3 CaCO3 + 2NaCl
111 g/mol 100.1 g/mol 58.4 g/mol
Mass Conversions in Sequences of Reactions
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Analogy: making cheese sandwiches
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CaCl2 + Na2CO3 CaCO3 + 2NaCl
5.45 g 4.55 g ? ?
How much CaCO3 (in g) is formed?
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