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8/3/2019 11 Intro to Stoich
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Quantitative Compositionof Compounds
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How do youcount things?
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What is a mole?
It is just a
representation of a number
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What is a mole?
Dozen = 12
mole = 6.022 x 1023
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How big is a mole?
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How big is a mole?
A mole of egg
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Mole
The mole is used when we're talkingabout numbers of atoms and molecules.
Atoms and molecules are very tinythings.
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Mole
A drop of water the size of the period at the end of this sentence would contain
10 trillion water molecules.
Instead of talking about trillions andquadrillions of molecules (and more),
it's much simpler to use the mole.
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Numerical Equivalent
Lorenzo Romano Amedeo Carlo
Avogadro di Quaregna e di Cerreto
6.022142 x 1023
6.022 x 1023
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We say,
A mole of a substance is the amount of the substance containing the same
number of atoms as there are carbonatoms in exactly 12 g of pure 12Cisotope.
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We say,
This represents the amount of thesubstance that contains 6.022 x 1023
particles (atoms, ions, and molecule).
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Solving for the number ofmoles
Number of moles x Avogadros No.=Number of particles
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Sample Problems
How many Na atoms are therein 0.125 moles of Na metal?
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Other Ways of solving for thenumber of moles
Atomic massMass in grams = MOLE (n)
Molar mass
Mass in grams= MOLE (n)
For elements
For compounds
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Sample Problems
How many moles of iron does 25.0grams of iron (Fe) represent?
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Sample Problems
How many magnesium atoms arecontained in 5.00 grams of Mg?
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Sample Problems
What is the mass of 3.01 x 1023 atomsof Na?
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Sample Problems
How many chlorine moleculesare present in 4.25 x 10-3
moles of Cl2?
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AtomicWeight/Mass
Atomic Weight is the relative weight of an atom expressed in amu (atomic
mass unit).The mass of its weighted averageatom (atomic mass units)
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AtomicWeight/Mass
Hydrogen, H, has a relative atomic weight of 1.0079 amu while carbon, C, has 12.011amu.
At. Mass = 7 (% nat. abun. x isotopic wt)
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MolecularWeight orMolar Mass
MolecularWeight of a substance isthe sum of the weights of the atoms as
shown in its formula.It is the mass of substance present permole.
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MolecularWeight orMolar Mass
Mol. Mass = 7 ( At. Wt. x # of atoms)
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Sample Problem
What is the mass of 1 mole of sulfuricacid?
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Sample Problem
How many moles of sodium hydroxideare there are there in 1.00 kg sodium
hydroxide?
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Sample Problem
How many molecules of hydrogenchloride are there in 25.0 grams of
hydrogen chloride?
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Percentage Composition
The amount an element in a compoundin percentage form
Molar mass of the compound
Total mass of the element =% composition x 100
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Sample Problem
Calculate the percentage composition of sodium chloride.
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Sample Problem
Determine the percentage compositionof ammonium acetate.
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Sample Problem
When heated in the air, 1.63 grams of zinc combines with 0.40 gram of
oxygen to form zinc oxide. Calculate thepercentage composition of thecompound formed?
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Empirical Formula andMolecular Formula
Empirical Formula is the simplest formula of a compound.
Gives the smallest whole-number ratioof atoms present in a compound
Gives the relative number of atoms of
each element in the compound
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Empirical Formula andMolecular Formula
Molecular Formula is a whole-numbermultiple of its empirical formula.
This is the true formula of thecompounds.
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Guidelines to Calculate theEF and MF
Determine the weight of each element in a compound.
Calculate the number of moles of theelements.
Determine the smallest mole.
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Guidelines to Calculate theEF and MF
Calculate the number of atoms of eachelement by dividing each mole of the
element by the smallest mole to get the empirical formula. (Should be awhole number)
Calculate the empirical formula weight
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Guidelines to Calculate theEF and MF
Compute for n.
Distribute n to the Empirical Formula toget the Molecular Formula
n = Molecular weightEmpirical Formula Weight
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Sample Problem
An unknown compound has thefollowing percentage composition,
% Ca = 36.04%
% Cl = 63.96%
What is the compound? (MW = 110)
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Sample Problem
Compound X was found to contain85.69% C and 14.31% H. The
molecular weight was determined to be55.9 amu. Calculate the empirical andmolecular formulas of X.
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Seatwork
Determine the molecular formula of thefollowing compounds given the
corresponding percent composition of each element. Name the compounds.
Compound Percent Composition
Molecular
Weights
ALi, 18.8; C, 16.2; O, 65.0 73.8
B K, 26.6; Cr, 35.3; O, 38.1 294.2