The Mole Concept and Atoms

Chapter 4

24 September 2013

Calculations and the Chemical Equation

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The Mole Concept and Atoms

– Unit of measurement for mass of an atom is atomic mass unit (amu) – unit of measure for the mass of atoms

• carbon-12 assigned the mass of exactly 12 amu

• 1 amu = 1.66 x 10-24_g

• What is the atomic weight of one atom of fluorine? Answer: 19.00 amu

• What would be the mass of this one atom in grams?

• Chemists usually work with much larger quantities

– It is more convenient to work with grams than amu when using larger quantities

Mass of Atoms

• A practical unit for defining a collection of atoms is the mole

1 mole of atoms = 6.022 x 1023 _atoms

The Mole

• To make this connection we must define the mole as a counting unit

– The mole is abbreviated mol

• A mole is simply a unit that defines an amount of something – Dozen defines 12 – Gross defines 144 – score 20 – ream 500

The Mole

F mol 1 F atom 10 022 . 6 F amu 1 F g 10 66 . 1 F atom 1 F amu 00 . 19 _  24 _  23 =19.00 g F/mol F or 19.00 g/mol F

Atomic Mass

• The atomic mass of one atomof an element corresponds to:

– The average mass of a single atom in amu

– The mass of a mole of atoms in grams

– 1atomof F is 19.00amu 19.00 amu/atom F

– 1 moleof F is 19.00g 19.00 g/mol F F mol 1 F atom 10 022 . 6 F amu 1 F g 10 66 . 1 F atom 1 F amu 00 . 19 _  24 _  23 =19.00 g F/mol F or 19.00 g/mol F note:

The name of the unit is “mole” The symbol is “mol”

Calculating atoms, moles, and mass

• We use the following conversion factors: • Density converts grams – milliliters

• Atomic mass unit converts amu – grams

• Avogadro’s number converts moles – number of atoms

• Molar mass converts grams – moles

Strategy for Calculations

• Map out a pattern for the required conversion • Given a number of grams and asked for

number of atoms

• Two conversions are required • Convert grams to moles

1 mol S/32.06 g SOR 32.06 g S/1 mol S

• Convert moles to atoms

Practice Calculations

1. Calculate the number of atoms in 1.7 moles of boron.

2. Find the mass in grams of 2.5 mol Na (sodium).

3. Calculate the number of atoms in 5.0 g aluminum.

4. Calculate the mass of 5,000,000 atoms of Au (gold)

Interconversion Between Moles,

Particles, and Grams

The Chemical Formula, Formula

Weight, and Molar Mass

• Chemical formula - a combination of

symbols of the various elements that make up the compound

• Formula unit - the smallest collection of atoms that provide two important pieces of information

– The identity of the atoms

– The relative number of each type of atom

Chemical Formula

• H₂– 2 atoms of hydrogen are chemically bonded forming diatomic hydrogen, subscript 2

• H₂O– 2 atoms of hydrogen and 1 atom of oxygen, lack of subscript means one atom

• NaCl – 1 atom each of sodium and chlorine

• Ca(OH)₂ – 1 atom of calcium and 2 atoms each of oxygen and hydrogen, subscript outside parentheses applies to allatoms inside

Chemical Formula

• (NH₄)₂SO₄– 2 ammonium ions and 1 sulfate ion – Ammonium ion contains 1 nitrogen and 4 hydrogen – Sulfate ion contains 1 sulfur and 4 oxygen

– Compound contains 2 N, 8 H, 1 S, and 4 O • CuSO₄._5H

2O

– This is an example of a hydrate- compounds containing one or more water molecules as an integral part of their structure

– 5 units of water with 1 CuSO₄

Homework: What does “sesqui” mean?

Comparison of Hydrated and

Anhydrous Copper Sulfate

Formula Weight and Molar Mass

of all atoms in the compound as represented by its correct formula

– expressed in amu

• What is the formula weight of H₂O? – 16.00 amu + 2(1.008 amu) = 18.02 amu

• Molar mass– mass of a mole of compound in grams / mole

– Numerically equal to the formula weight in amu

• What is the molar mass of H₂O? – 18.02 g/mol H₂O

Formula Unit

• Formula unit – smallest

collection of atoms from which the formula of a compound can be established

• When calculating the formula weight (or molar mass) of an ionic compound, the smallest unit of the crystal is used

What is the molar mass of (NH₄)₃PO₄? 3(N amu) + 12(H amu) + P amu + 4(O amu)=

Molar Mass

• Molar mass - The mass in grams of 1 mole of atoms

• What is the molar mass of carbon? 12.01 g/mol C

• This means counting out a mole of carbon atoms (i.e., 6.022 x 1023_{) they would have a mass}

of 12.01 g

• One mole of anyelement contains the same

number of atoms, 6.022 x 1023_{, Avogadro’s number}

The Chemical Equation and the

Information It Conveys

A Recipe For Chemical Change

• Chemical equation- shorthand notation of a chemical reaction

– Describes all of the substances that react and all the products that form, physical states, and experimental conditions

Features of a Chemical Equation

specified using chemical symbols

2. Reactants are written to the left of the reaction arrow and products are written to the right 3. Physical states of reactants and products may

be shown in parentheses

4. Symbol  over the reaction arrow means that energy is necessary for the reaction to occur 5. Equation must be balanced

) ( O ) 2Hg( ) 2HgO(s  l  ₂ g Products

Products – written on the right Reactants – written on the left of arrow

Products and reactants must be specified using chemical symbols

 – energy is needed

“Everything” is balanced

•number of atoms

•moles of stuff

•mass

•energy

•charge