atomic weights

Atomic Weights The mass of an individual atom, ion, or

molecule is very small.

Scientists use the atomic mass unit (amu) to express the mass of atoms or molecules.

1 amu = 1.66054 x 10-24 g

1 g = 6.022 x 1023 amu

Atomic Weight Atomic weight (atomic mass):

the average mass of one atom of an element expressed in amu

found on the periodic table

The atomic weight of an element is reported as the weighted average of the naturally occurring isotopes.

Atomic Weights The atomic weight for silicon is found by

taking a weighted average of the masses of the three naturally occurring isotopes of silicon:

28Si : 92.2297% of 27.9769 amu = 25.803 amu

29Si : 4.6832% of 28.9765 amu = 1.3570 amu

30Si: 3.0872% of 29.97377 amu = 0.92535 amu

AW of Si = 28.085 amu

Formula Weight The mass of a compound can be

expressed using either its formula weight or its molecular weight (molecular compounds only).

Formula Weight: the sum of the atomic weights of all of

the atoms in any chemical formula

Molecular Weight: the sum of the atomic weights of all of

the atoms in a molecular formula

Formula Weight and Molecular Weight

Formula weight and molecular weight are found by the exact same process:

The molecular weight of CO2 is calculated as follows:

FW = 1 (AW of C) + 2 (AW of O) = 1 (12.01 amu) + 2

(16.00 amu) = 44.01 amu


The formula weight of CaCl2 is calculated as follows:

FW = 1(AW of Ca) + 2(AW of Cl)

Ca: 1 (40.08 amu) = 40.08 amu

Cl: 2 (35.45 amu) = 70.90 amu

110.98 amu


How many significant figures should I use for the atomic weight???

Use enough significant figures in the AW to avoid limiting the number of significant figures in any other calculation.

I want you to always use the AW’s rounded appropriately to either + 0.1 or 0.01 amu.

Molecular Weight

Example: Calculate the MW of phosphoric acid

Phosphoric acid = ????

Mole

Atoms and molecules are very small.

1 cm length20 to 50 million atoms

Tiny speck of carbon60 million billion atoms (6 x 1016)

1 cm3 boxbillions of trillions of atoms

Mole Virtually impossible to manually count

the # of atoms in a reasonable size sample Tiny speck of carbon

60 million billion atoms (6 x 1016)

Counting 1 atom per second, 24 hours per day, 365 days per year it would take:

~ 2 billion years

Mole

Knowing the number of atoms or molecules in a sample is important. Chemical reactions occur on an atomic

or molecular level.

2 H2 + O2 2 H2O

H H

O O+

H H

Mole

Chemists usually measure out chemicals in grams not as individual atoms or molecules.

Need a way to count the number of atoms or

molecules in a sampleand

relate the number of atoms or molecules to the mass of a sample

Mole

Mole: the amount of a material that contains

the same # of units as there are atoms in exactly 12 g of C-12

the amount of material that contains 6.022 x 1023 atoms (or molecules or ions or any other chemical species)

Mole

1 mole 12C atoms = 6.022 x 1023 12C atoms

1 mole H2O molecules = 6.022 x 1023 H2Omolecules

1 mole Na+ ions = 6.022 x 1023 Na+ ions

Mole Simply a way to count, express, and

compare the number of atoms or molecules present

pair of gloves (2) dozen eggs (12) case of pop (24) gross (144)

Mole (contains 6.022 X 1023 units)

Avogadro’s Number

Avogadro’s Number (N): the number of atoms, ions, or molecules per mole

a constant

N = 6.022 x 1023 atoms, ions or molecules

mole

Avogadro’s Number

How big is Avogadro’s Number?

If you have a mole of pennies (i.e. Avogadro’s number) and divide them equally among the 6 billion people on Earth, how many dollars would each person get?

Avogadro’s Number

6.022 X1023 pennies 6 x 109 people = $1 x 1012

person

x $1 100 pennies

Each person would get ~ $ 1 trillion!!!

Comparing moles of different materials

1 mole Cu = 6.022 x 1023 atoms = 63.55 g Cu

22 copper (pre-1983) pennies


1 mole He = 6.022 x 1023 atoms = 4.003 g He

~7 medium-sized He-filled ballons


1 mole H2O = 6.022 x 1023 molecules of H2O = 18.016 g H2O

~18 mL (1.2 T) of water

Mole

How do we relate the number of atoms (or molecules) in a mole to mass???

1 mole = 6.022 x 1023 atoms

The mass of 6.022 x 1023 atoms is a unique number for each element. Atoms of each element have different

masses1 mole of each element has a unique mass (molar mass)

Molar Mass Molar mass

the mass in grams of 1 mole of substance

numerically equal to its formula weight (for ionic compounds), molecular weight (for molecules), or atomic weight (for atoms)

Molar Mass

1 H2O molecule weighs 18.0 amu

1 mol H2O weighs 18.0 g

1 NO3- ion weighs 62.0 amu

1 mol NO3- ions weighs 62.0 g

1 mole of N2 weighs 28.0 g

28.0 g N2 1 mole N2

1 mole N2 28.0 g N2

Molar Mass Molar mass can be used as a conversion

factor: Relates the # grams and # moles

OR

Calculating the number of moles in a sample

Example: A medium-sized balloon contains 0.55 g of He gas. How many moles of He are in the balloon?

Calculating the # of moles

Example: How many moles are in 25.0 g of acetic acid ?

Calculating mass from moles

Example: A 1-carat diamond, which is pure carbon, contains 0.0167 mol of carbon. What is the mass, in grams, of the diamond?

Calculating mass from moles

Example: What is the mass in grams of 0.500 moles of carbon dioxide?

Avogadro’s Number Avogadro’s number can also be used as

a conversion factor:

1 mole = 6.022 x 1023 molecules (oratoms, ions, formula units)

6.022 x 1023 atoms 1 mole1 mole 6.022 x 1023 atomsOR

Calculating # of molecules from # of moles

Example: How many water molecules are in 2.5 moles of water?

Calculating # moles from # of molecules

Example: How many moles are in 3.2 x 1022 molecules of water?

Calculating Mass from # of Molecules or Vice Versa

grams molesUse

molar mass

molecules

Use Avogadro’s

number

Calculating mass from molecules

Example: What is the mass in grams of 3.0 x 1024 molecules of nitric acid?

Calculating # of molecules from mass

Example: How many molecules are in a 5.0 g sample of hydrogen?

atomic weights

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