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Atomic Weight, Molecular Weight, Formula Weight

and Molar Mass

loosely based on Chap 3 Sec 1of Brady Senese

- Dr. C. Yau
- Fall 2009

1

1

Atomic Weight or Atomic Mass

- Examine the periodic table. What is the atomic

weight (AW) of carbon?

The atomic weight of C is 12.01 amu/atomor

12.01 amu/atom

It means that each C atom weighs12.01 u (or

12.01 amu). (In this class you should make it a

habit to look up AW to 4 sig. fig.)

2

Molecular Weightor Molecular Mass

The AW of C is 12.01 amu/atom The AW of O is

16.00 amu/atom. Therefore, the mass of one

molecule of CO2 would be 12.01 2(16.00) 44.01

amu/molecule. (Always round to 4 sig. fig.) We

call this the molecular weight (MW) or molecular

mass. What is the MW of methane (CH4)?

Ans. 4.032 amu/molecule

3

3

Formula Mass

If we add up the atomic mass of Na and Cl we get

22.99 35.45 58.44. What is wrong with

saying the molecular weight of NaCl is 58.44

amu/molecule?

4

4

Formula Mass

For molecular substances, the smallest unit is

the molecule. For ionic substances, the smallest

unit is the "formula unit" abbreviated f.u. e.g.

If you pluck out the smallest unit out of water,

you would have a water molecule.

5

5

Formula Mass

When you pluck out the smallest unit of table

salt you would have a formula unit of NaCl.

NaCl-

NaCl?NaCl?NaCl? Cl?NaCl?NaCl?Na NaCl?NaCl?

NaCl?

We often simply write NaCl, but it is understood

that Na is Na and Cl is Cl-.

There are no molecules in an ionic compound!

6

6

AW, MW and FW

- Summary
- Atomic Weight (AW) mass of one atom
- (for atoms)
- in units of amu/atom
- Molecular Weight (MW) mass of one molecule
- (for molecular substances)
- in units of amu/molecule
- Formula Weight (FW) mass of one f.u.
- (for ionic compounds)
- in units of amu/f.u.

7

Calculations of AW, MW and FW

- Calculate the formula weight of
- (and give the proper units)
- cobalt(III) sulfide
- ammonium sulfate
- Calculate the molecular weight of
- (CH3CH2)3N

8

- Calculate the formula weight of
- Na2SO4?2H2O
- This is known as a hydrate.
- The dot in the formula does NOT mean you multiply

anything. - The dot means there are 2 water molecules

attached to each f.u. of Na2SO4. - What is its formula weight?

9

The Mole Concept

One mole is defined as the number of atoms in

exactly 12 g of C-12 atoms. Remember that C-12

refers to the isotope.

exactly 12 g C

6.022x1023 C atoms

It turns out that there are 6.022x1023 C atoms in

those 12 grams. So. 1 mole 6.022x1023(not an

exact number, but in 4 sig. fig.)

MEMORIZE THIS NOW!

10

10

The Mole Concept

- 1 mole 6.022x1023
- This number is known as
- Avogadro's number
- Note Mole is abbreviated mol
- NOT m (which stands for meters)
- Note It is a NUMBER.
- It is NOT a mass.
- It does NOT have the unit of grams.

11

The Mole Concept

- We all know
- 1 dozen eggs 12 eggs
- 1 dozen atoms 12 atoms
- In the same way, we use the word "mole"
- 1 mole eggs 6.022x1023 eggs
- Obviously that is a VERY big number of eggs.

There would be no occasion to be talking about a

mole of eggs. - When do you we use "moles?"

12

The Mole Concept

1 mol atoms 6.022x1023 atoms 1 mol molecules

6.022x1023 molecules 1 mol electrons 6.022x1023

electrons The "mole" is not a mass itself, just a

number of items. Why do we bother with such a

strange number? Let us take a look at the

periodic table.

13

13

The Mole Concept

The atom is a very, very small particle. You can

fit 7000 atoms on the dot of an i. It is too

small to see with the naked eye, and too small to

hold in your hand or weigh on the balance. We

must use a much larger quantity of atoms a mole

of atoms.

14

14

The Mole Concept

From the periodic table, we can get the atomic

weight for Ag. This mass can be given in two

units 107.9 amu per atom or 107.9 g per mole of

atoms

One atom of silver weighs 107.9 amu, which is

0.0000000000000000000001079 gram Get the picture?

You cannot weigh a single atom of silver, but you

CAN weigh a mole of Ag atoms. 107.9 g is about

3.8 ounces.

15

15

The Mole Concept

From the periodic table we see that Ag is 107.9

g/mol Cu is 63.55 g/mol What does 107.87 g of Ag

have in common with 63.55 g of Cu? They both

contain the same number of atoms 6.022x1023 Ag

atoms and 6.022x1023 Cu atoms, in other words, 1

mole Ag and 1 mol Cu.

16

16

The Mole Concept

Earlier we were talking about the AW, MW and FW.

Now we are going to add one more. MM molar

mass Whenever you see "molar" you should think

"per mole." Molar mass mass per mole (for 1

mol) with units of

g/mol KNOW THIS WELL! SEE THIS IN YOUR

DREAMS.AND NIGHTMARES!

17

17

Molar Mass (MM)

The atomic mass from the periodic table can be

used with two units amu/atom or g/mole (or g

mol-1) REMEMBER Whenever you see a fractional

unit (such as amu/atom, g/mol, g/mL) you should

think "conversion unit"! The g/mol gives you the

link between MASS AND of MOLES.

18

18

Molar Mass (MM)

The FW ofCo2(SO4)3 is 406.7 amu/f.u. Its MM

406.7 g/mol 2(58.93)3(32.07)12(16.00)

406.7 We had calculated the MW of (CH3CH2)3N

to be 101.2 amu/molecule Its MM 101.2

g/mol 2x3C 5x3H N 6(12.01) 15(1.008)

14.01 101.2

19

19

Molar Mass (MM)

Note that we no longer need to distinguish

between f.u. and molecules in the units of

MM. It is understood that for Co2(SO4)3 its MM

406.7 g/mol of f.u. and for (CH3CH2)3N its MM

101.2 g/mol of molecules.

20

20

Use of MM in Dimensional Analysis

Conversion factors from MM e.g. MM (N)

14.01 g/mol

e.g. MM (HCl) 36.45 g/mol

21

21

Use of MM in Dimensional Analysis

When do we use MM?

Whenever you see a fractional unit (such as g/mol

or g mol-1) you should think "conversion

factor"! MM is the link between mass moles

mass moles of particles

moles of atoms moles of f.u. moles of

molecules e.g. mol Cu mol CuCl2 mol PBr3

22

22

Use of MM

- Example 3.1 p.89
- How many moles of sulfur are there in a 23.5 g

sample of sulfur? - Example 3.2 p.89
- We need 0.254 mol of iron(III) chloride for a

certain experiment. How many grams do we need to

weigh? - Do Practice Exercises 1 2 on p.90

Use of Avogadro's Number in Dimensional Analysis

Conversion factors from Avogadro's number

Note that X can be anyTHING, such as CO2, K2SO4,

electrons, Na, etc. as long as it is EXACTLY

the same on top as on the bottom. BUT it cannot

be a unit, such as g or mL.

24

24

Use of Avogadro's Number in Dimensional Analysis

You use Avogadro's number ONLY if you have to go

between the macroscopic and the particulate

(sub-microscopic) levels.

particulate level macrosopic level

(conceptual, (observable) non-observable)

atoms moles of atoms f.u. moles of

f.u. molecules moles of molecules amu g, mg, lbs,

oz

25

25

Use of Avogadro's Number

- Example 3.3 p.90
- How many atoms of copper are there in a piece of

pure copper wire that weighs 14.3g? - Example 3.4 p.91
- What is the mass in grams of one molecule of

carbon tetrachloride (CCl4)? - Example
- What is the mass in grams of 2.01 mole of NaCl?

Use of Avogadro's Number

- Example
- How many anions are in 7.82 g of MgCl2?
- Example
- What is the weight in grams of 158 molecules of

CO2? - REMEMBER! ASK YOURSELF
- DO I NEED AVOGADRO'S NUMBER IN THIS PROBLEM?
- Do Practice Exercises 3 4 on p.91

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