Unit 3:Atomic Theory

Law of Conservation of Mass

• Mass is neither created nor destroyed during chemical or physical changes

• The total mass in the reactants has to be equal to the total mass of the products

• Atoms are rearranged

John Dalton

• 1808- proposed an explanation for the law of conservation of mass

Dalton’s Atomic Theory: (p 66)

1. All matter is composed of extremely small particles called atoms.2. Atoms of a given element are identical in size, mass and other properties: atoms of different elements differ in size, mass and other properties3. Atoms cannot be subdivided, created or destroyed

Dalton’s Atomic Theory: (con’td)

4. Atoms of different elements combine in simple whole number ratios to form chemical compounds5. In chemical reactions, atoms are combined, separated and rearranged.

Which aspects of Dalton’s theory are incorrect?

• Atoms are divisible into smaller particles

• The same element can have different masses ( isotopes)

• The Modern Atomic Theory maintains Dalton’s points and accounts for the corrections stated above.

Law of Definite Proportions

• a chemical compound contains the same elements in exactly the same proportion by mass regardless of size of the sample

• Ex. NaCl 39.34% Na and 60.66% Cl ALWAYS!

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Law of Multiple Proportions• If two or more different compounds are formed

of the same two elements, the ratio of the masses of the second element combined with a certain mass of the first is always a ratio of small whole numbers

• Ex. CO 1.0g C to 1.33 g O• CO2= 1.0 g C to 2.66 g O• Ratio between the 2 oxygens is two!

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Law of Multiple proportions continued…..

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The Mole

• SI unit for the amount of substance• It’s a counting unit

– 1 dozen = 12 units– 1 pair = 2 unit– 1 gross = 144 units– 1 MOLE = Avogadro’s # = 6.02 x 1023

• Why do we need such a huge number in chemistry?

The Mole in perspective

• If you have a mole of pennies, and distribute them equally among every person on Earth, how much $$ would everyone get?

The Mole in perspective

• If you have a mole of pennies, and distribute them equally among every person on Earth, how much $$ would everyone get?

The Mole in perspective• 1. One mole of paper would make a stack that

would reach to the moon more than 80 billion times

•2. One mole of grains of sand would be more than all of the sand on Miami Beach

•3. One mole of blood cells would be more than the total number of blood cells found in every human on earth

• 4. A one liter bottle of water contains 55.5 moles of water

5. One mole of seconds is about 19 quadrillion years, 4,240,666 times the age of the earth, or 954,150 times the age of the universe itself

•6. One mole of cents could repay the United States National Debt 86 million times

Avogadro’s Number:

• Italian scientist, Amadeo Avogadro• Number of particles in a mole is

known as Avogadro’s number = 6.02 X1023

Molar Mass:

• The mass of one mole of pure substance

• Ex. Lithium = 6.94 g/mol

Molar conversions:

Calculate the molar mass of:

• Cu• NaCl• Ca2SO4

• Al(NO3)3

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Moles & Mass

For Calculations - Use Dimensional Analysis – Set up

• What is the mass of 3.0 mols of NaCl?

• How many mols are in 30.0 g of H2O?

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Moles & Particles

• How many atoms are in 3.0 mols of H2O?

• How many moles of Aluminum, are in 4.02 x 1023 atoms of Al?

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Moles & Particles & Mass

• Calculate the mass of 3.7 x 1024 molecules of MgCl2.

• How many atoms are in 100.0g of Silver?

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Do Now

• How many particles are in 200g of Gold?

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Molarity (M)

• Number of moles of solute in one liter of solution

• Molarity = (moles of solute/ L of solvent)

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Percent Composition• Percent by mass of each element in a

compound.• % composition of any pure substance is

always constant, whatever the source or sample size.

• Useful in identifying the characteristic of any substance.

• Used by investigators from many fields, Analytical chemistry, forensics etc. to determine the identity of unknown materials.

Sample Problems

• Calculate the percentage composition of each element in NaNO3.

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Answer

• 27.05%Na, 16.48% N, 56.47% O

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Sample Problem 2

Calculate the percentage composition of Oxygen in Ag2SO4.

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Answer

• 69.19% Ag, 10.28% S, 20.52% O

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Water in a hydrate

• What is the mass percentage of water in the hydrate CuSO4●5H2O?

• 36.08%

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Determining Chemical Formulas

• Empirical formulas – consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number ratio of the different atoms in the compound.

• Molecular Formulas – a multiple of the empirical formula.

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Guidelines

• Determine the mass of each element

• Convert into moles• Divide by the smallest mole or the

elements present• Determine the ratio• Write the formula (Usually

elements are listed from left to right on the PTOE)

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Sample Problems

• A compound is analyzed and found to contain 36.70% K, 33.27% Cl, and 30.03% O. What is the empirical formula of the compound?

• A 60.00 g sample of tetramethyllead, a gasoline additive, is found to contain 38.43 g of lead, 17.83 of carbon, and hydrogen. Find its empirical formula.

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