atoms, moles, and the mole concept

8/8/2019 Atoms, Moles, And the Mole Concept

1/17

` Atomic weight this is a number thatexpresses the mass of one atom of anelement relative to an atom of C-12.

`

C-12 has been chosen as the standard andassigned a mass of exactly 12 atomic massunits (a. m. u.).

` Fluorine has an atomic weight of 19 whichmeans that one atom of fluorine is 19/12 as

heavy as one atom of C-12.` Hydrogen has an atomic weight of 1 which

means that one atom of hydrogen is 1/12 asheavy as C-12.


2/17

` M olecular weight refers to the mass of onemolecule of a substance relative to the mass of one atom of C-12.

` The molecular weight of a molecule is determinedby adding the atomic weights of all the atoms inthe formula for the substance as given in themolecular formula.


3/17

` For example: H 20 has 2 atoms of hydrogen and 1atom of oxygen.

` The molecular weight would be:

2(at. wt. H) + 1(at. wt. O) = MW H202(1) + 1(16) = 18Therefore, one molecule of H 20 is 18/12 as heavy as oneatom of C-12.


4/17

` S ince atoms and molecules are extremely smalland have extremely small masses, working withone or even a dozen in the laboratory is very

unrealistic.` Chemists had to come up with a convenient way

to count atom, molecules, and formula units.


5/17

The M ole Concept


6/17

` If we count 12 eggs we say we have a dozen.` If we count 144 pencils we say we have a

gross.` A ream of paper implies 500 sheets.` But how do we count atoms or molecules?

We cant measure or weigh a dozen atoms or agross of molecules!


7/17

` Atoms are so small that to have enough of them to weigh on the scale requires anenormous number of them!

That means we need our own counting system in

Chemistry.The term we use to indicate amount is called themole .

` It just represents a number (albeit a very large number!)


8/17

` One mole of atoms is 6.02 x 10 23 atoms.That is 602,000,000,000,000,000,000,000 atoms

That number is too large for us to comprehend withoutsomething tocompare it to.

One mole of marbles wouldhave a volumethe size of the

moon!


9/17

` One mole of Olympic shotput balls wouldequal the mass of the earth!


10/17

A p article can be anythingmicrosco p ic, like a moleculeor an atom.

` Avogadros number is 6.02 x 10 23

` W e express it mathematically as1 mole = 6.02 x 10 23 particles

` This would be similar to1 dozen = 12 or 1 gross = 144


11/17

` The mass given on the Periodic Table hasunits of amu if we are speaking of an atom or a molecule.

` If we are speaking of a mole of atoms or molecules, the mass is in terms of grams !

` This allows us to weigh the mass of elementsand compounds on the balance.

Our balance does not measure in terms of amus!


12/17

1 atom C = 12.01 amu 1 mole C = 12.01 g

1 molecule O 2 = 32.00 amu 1 mole O 2 = 32.00 g

1 molecule CO 2 = 44.01 amu 1 molecule CO 2 = 44.01 g

1 molecule Na 2S

O 4 = 142.1 amu 1 molecule Na 2S

O 4 = 142.1 gN otice that the numerical value of the various

atomic and molecular masses are identical, but theunits are very different.

One mole of any element or com p ound has the samemass in grams that one atom or molecule of thesame substance has in amus.


13/17

` This represents the mass (in grams)of one mole of any substance. This informationis right on the Periodic Table

` Lets do the M olar M ass ( MM ) of NaCl:1 mol of Na = 22.99 g1 mol of Cl = 35.45 gThe molar mass is

22.99 g + 35.45 g = 58.44 g/mol

Notice the units for M olar M ass!


14/17

` W hat is the molar mass of (NH 4)3PO 4?How many N?

x 3How many H?

x

3 x 4 = 12How many P?

x Only 1How many O?

x 4

This gives a molar mass of x (3 x 14.01) + (12 x 1.008) + (1 x 30.97) + (4 x 16.00)

= 149.1 g/mol


15/17

` W hat is the mass of 4.5 x 10 24 atoms of carbon(in g)?

S tart with what has been given and then look for waysto go from units of atoms to units of grams.

N otice that Avogadros number is just a conversion factor!

244.5 x 10 atoms C23

| 1 mol C| 6.02 x 10 atoms

| 12.01 g C=

| 1 mol C89.78 g C


16/17

` W hat is the mass (in g) of 3.60 x 10 22 molecules of O 2 gas?

S tart with what has been given and then look for ways to go from units of molecules to units of grams.

N otice that Avogadros number is still just a conversion factor!

2223.60 x 10 molecules O 2

23

| 1 mol O| 6.02 x 10 molecules

2

2

| 32.00 g O=

| 1 mol O21.91 g O


17/17

` How many HF molecules are present in1.5 x 10 3 g of hydrogen fluoride gas?

S tart with what has been given and then look for ways to go from units of grams to units of molecules.

N otice that Avogadros number isagain just a conversion factor!

31.5 x 10 HF 1 HF0.01 HF

3| 6.0 x 10 ecu es HF

| 1 HF5

.5 x 10 ecu es HF

atoms, moles, and the mole concept

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