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Molecular Mass &

Mr. Shields Regents Chemistry U04 L01

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Atomic Mass Units

Recall all Atomic masses are based on Carbon-12

- C-12 is Defined to be equal to EXACTLY 12 amu’s

- 1 amu = 1/12 the mass of carbon-12

- This defines a SCALE that is used to specify the mass of all other elements relative to C-12

Note: 1) Atomic mass and Mass # are different. 2) Atomic mass is also called atomic wt.

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Atomic Mass Units

Using this scale, C-12 = exactly 12.000 amu but…

- The Atomic mass of Chlorine-35 = 34.969 amu The Atomic mass of Chlorine-37 = 36.966 amu

- The Atomic mass of each element is the weighted average of all it’s isotopes

- For Ex. the Atomic mass of Chlorine is 35.453 and the atomic mass of Carbon is 12.011

- i.e the weighted avg of C-12, C-13, C14

Weighted Avg

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Molecular Compounds

Do you remember the definition Compounds?

- Two or more elements joined together by chemical bonds

- Compounds can exist in either of two forms:

MOLECULES or IONIC COMPOUNDS

-Molecules are a non-repeating single neutral group of atoms covalently bonded together (this means diatomic elements are also molecules)

Ex. of molecules: N2, C6H12O6, C6H14, C2H5OH, CO2, or H2O

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IONIC COMPOUNDS

Ionic compounds are REPEATING representative units of Positive (cations) and Negative ions (anions) joined togetherto form a neutral compound

=

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Representative Unit

The unit in a compound depends on the structure of the substance.

For covalently bonded substances, the representative unit is the molecule (non-repeating).

For ionic bonded atoms, which make a crystal lattice, the representative unit is the repeating formula unit, ex. NaCl

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Molecular Mass

Let’s look at a molecule of water …

How many H atoms are there and what is it’s totalAtomic mass ?

2 Hydrogen = 1.008 amu x 2 = 2.016 amu

How many O atoms are there and what’s it’s totalAtomic mass?

1 Oxygen = 15.999 amu

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Molecular Mass

Now let’s add them together:

2 Hydrogen = 2.016 amu + 1 Oxygen = 15.999 amu

18.015 amu

The sum of all the masses of the atoms in a molecule expressed in amu’s is called the MOLECULAR MASS

Calculate the molecular mass of C6H1486.18

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Formula Mass

Remember … Molecules exist assingle entities while Ionic compoundsexist as Repeating units of Positive & Negative ions in a crystal lattice.(so NaCl is actually NaxClx wherex= some large number).

Molecules can thus be easily & accurately represented byThe molecular mass ( ex. Sugar - C6H12O6 = 180.15 amu)

Whereas IONIC (Crystalline) compounds can not. Why?

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Formula Mass Problems

What is the FORMULA MASS for CaF2 ?

(Ca) 40.08 + (2F) 38.00 = 78.08 amu

Instead of Molecular mass we need to use somethingcalled FORMULA MASS.

Formula mass is the mass of one single repeating unit.

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AMU to Mass

Up to now we’ve discussed Atomic mass in terms of AMU’s.

How can we relate AMU’s to something we can weigh inthe Laboratory, like grams?

Let’s consider the ATOMS 1H and 14N

a. 1 atom of 1H = 1 amu (rounded)b. 1 atom of 14N = 14 amu (rounded)

c. What is the ratio of AMU’s?     1 : 14

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AMU to Mass

What happens to the ratio if we increase the number of1H and 14N atoms equally? Nothing. Let’s take a look …

No. of Atoms

Mass of H (amu’s)

Mass of N (amu’s)

Ratio of amu’s

1 1 14 1:14

5 5 70 1:14

10 10 140 1:14

100 100 1400 1:14

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AMU to Mass

When comparing elements, as long as the ratio of amuis held constant then the number of atoms present For each element must be equal.

THE NEXT QUESTION YOU MIGHT ASK IS…

How many atoms would be necessary to equal in grams an atoms atomic mass in amu’s?

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AMU to Mass

For example the Atomic mass of Sn is 118.71 amu’s

How many atoms are needed so that all the atoms collectively weigh 118.71 grams?

From this the concept of the MOLE was eventually developed.

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AMU to Mass

Scientists have experimentally determined that forC-12 the number of atoms needed to equate to 12 amu to 12 grams is:

6.02 X 1023 (1 mole)

or 602,000,000,000,000,000,000,000 602,000,000,000,000,000,000,000 AtomsAtoms

A mole of any element is defined as the number of atomsin exactly 12.0 grams of Carbon-12.

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AMU to MASS

Recall that for any two elements there would alwaysbe an equal number of atoms of each as long as the ratioof atomic masses remained constant

 

# of atoms Mass of C-12 Mass of N-14 Ratio of C to N masses

1 12 amu 14 amu 1 : 1.167

6.02 x 1023

(1 mole)12 g 14 g 1 : 1.167

This means there are 6.02 x 1023 atoms in EVERY ATOMIC MASS (amu) value when expressed in grams !

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If there are 6.02 x 1023 atoms in 12 grams of C-12 howMany atoms are there in 6 grams of C-12?

3.01 x 1023 atoms

Problem:

How many grams are there in 5.03 x 1023 atoms of Zn?(Remember: 6.02 x 1023 = # atoms needed to express amu’s as grams)

(5.03 x 1023/6.02 x 1023) x Atomic Mass = 0.836 moles x 65.38 amu = 54.63g

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Avogadro's number = 6.02214199 × 1023

atoms/mole

6.02 x1023/1x107 = 6.02x1016 sec or about 2 billion years

For example:If a computer could count 10 million atoms per second how longwould it take to count 1 mole of atoms?

This is usually designated as NA

Amadeus Avogadro

Today we know what this numberof atoms is to a high degree ofAccuracy and it has a name

This is a really BIG number!

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Remember …

Molecular Mass is used for moleculesAnd is expressed in AMU’s

Formula Mass is used for Crystalline (ionic)compounds and is also expressed in AMU’s

And If we express an element or compounds mass in gramsIt is called …

The Gram Molecular Mass or the Gram Formula Mass