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