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Atomic Weight, Molecular Weight, Formula Weight and

Molar Mass

Dr. C. YauSpring 2014

loosely based on Chap 4 Sec 1of Jespersen 6th edition

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Examine the periodic table. What is the atomic weight (AW) of carbon?

Atomic Weight or Atomic Mass

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01.12C

The atomic weight of C is

or 12.01 u/atom

It means that each C atom weighs12.01 u.

(In this class you should make it a habit to look up AW to 4 sig. fig.)

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Molecular Weightor Molecular Mass

The AW of C is 12.01 u/atom

The AW of O is 16.00 u/atom.

Therefore, the mass of one molecule of CO2 would be 12.01 + 2(16.00) = 44.01 u/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. 16.04 u/molecule

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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 is58.44 u/molecule?

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Formula MassFor 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.

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Formula MassWhen you pluck out the smallest unit of table salt you would have a formula unit of NaCl.

Na+ClNa+ClNa+ClClNa+ClNa+ClNa+

Na+ClNa+ClNa+Cl

Na+Cl-

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!

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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.

AW, MW and FW

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Calculate the formula weight of…(and give the proper units) cobalt(III) sulfide

ammonium sulfate

Calculate the molecular weight of… (CH3CH2)3N

Calculations of AW, MW and FW

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Calculate the formula weight of…

Na2SO42H2O

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?

101010

The Mole ConceptOne mole is defined as the number of atoms in exactly 12 g of C-12 atoms.

Remember that C-12 refers to the isotope….

C126

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!

6.022x1023 C atoms

exactly 12 g C

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1 mole = 6.022x1023

This number is known asAvogadro'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.

The Mole Concept

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We all know…1 dozen eggs = 12 eggs1 dozen atoms = 12 atomsIn the same way, we use the word

"mole"…

1 mole eggs = 6.022x1023 eggsObviously 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?"

The Mole Concept

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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.

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The Mole ConceptThe 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.

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The Mole Concept

One atom of silver weighs 107.9 u, 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.

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87.107Ag

From the periodic table, we can get the atomic weight for Ag. This mass can be given in two units:

107.9 u per atom

or

107.9 g per mole of atoms

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The Mole ConceptFrom 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.

Cu Ag

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The Mole ConceptEarlier 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!

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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 factor"!The g/mol gives you the link betweenMASS AND # of MOLES.

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Molar Mass (MM)The FW ofCo2(SO4)3 is 406.7 amu/f.u.Its MM = 406.7 g/mol2(58.93)+3(32.07)+12(16.00) = 406.7

We had calculated the MW of… (CH3CH2)3N to be 101.2 amu/moleculeIts MM = 101.2 g/mol2x3C + 5x3H + N = 6(12.01) + 15(1.008) + 14.01 = 101.2

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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)3its MM = 406.7 g/mol of f.u.and for (CH3CH2)3N its MM = 101.2 g/mol of molecules.

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Use of MM in Dimensional Analysis

Conversion factors from MM: e.g. MM (N) = 14.01 g/mol

HCl g 36.46

HCl mol 1or HCl mol 1HCl g 46.36

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

N g 14.01

N mol 1or N mol 1N g 01.14

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Use of MM in Dimensional Analysis

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

When do we use MM?

mass moles of particles moles of atoms

moles of f.u.moles of moleculese.g. mol Cu mol CuCl2

mol PBr3

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Use of MMExample 4.1 p.109Titanium(IV) oxide is one of the best

sunscreens because it completely blocks ultraviolet radiation from reaching the skin. In an experiment to prepare TiO2, we start with 23.5 g sample of titanium. How many moles of Ti do we have?

Example 4.2 p.110We need 0.254 mol of iron(III) chloride for

a certain experiment. How many grams do we need to weigh?

Do Practice Exercises 4.1 & 4.2 on p.110(Note: Ans to Pract. Exercises are at the back of the

book.)24

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Use of Avogadro's Number in Dimensional Analysis

Conversion factorsfrom Avogadro's number:

X mol 1X 6.022x10or

X 6.022x10

X mol 1 23

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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.

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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 atomsf.u. moles of f.u.molecules moles of moleculesu g, mg, lbs, oz

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Use of Avogadro's NumberExample 4.3 p.111Tungsten wire is the filament inside most

incandescent light bulbs. In a typical light bulb, the tungsten filament weighs 0.635 g. How many atoms of tungsten are there in such a light bulb flame?

First ask yourself... “Do I need Avog. #?”

Example 4.4 p.112Carbon tetrachloride was used as a dry-

cleaning fluid until it was found to be carcinogen. What is the average mass in grams of one molecule of carbon tetrachloride (CCl4)?

“Do I need Avog. #?”

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Use of Avogadro's NumberExampleWhat is the mass in grams of 2.01 mole of NaCl?

ExampleHow many anions are in 7.82 g of MgCl2?

ExampleWhat 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 4.3 & 4.4 on p.112