Notes Molar Mass_percent Composition_Molecular Formulas

7/30/2019 Notes Molar Mass_percent Composition_Molecular Formulas

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Chapter 6 Notes Page 1 Updated 12/9/2012

Chapter 6: Moles, Molar Mass, Percent Composition and

Formulas

6.1 Atoms and Moles

1) Avogadros Number

a) Memorize: 6.022 x 1023

b) Avogadro discovered that there are 6.022 x 1023 atoms

in 1 gram of hydrogen.

2) Be able to explain and use the concept of the mole

a) Avogadro called his number a mole of matter.

b) The word moleis just like the word dozen. Dozenmeans 12. You can havea dozen of anything. You can also have a mole of anything.

3) How big is a mole?

a) Here it is written out: 602,200,000,000,000,000,000,000

b) Thats 602 billion groups of a trillion!

c) Lets just do an example with paper clips.

d) If you have a mole of paper clips and made them into a chain, how many times

could you go to the moon and back with your chain? (You dont need to do this)

4) Convert moles to # of atoms

a) How many atoms are in 3.2 mol potassium (K)?

b) Remember: 1 mol = 6.02 x 1023 atoms

c) This can be written as a conversion factor:

5) How do we use the Mole in chemistry?

a) The atomic mass of an element is the grams of 1 mole of that atom

b) Why do chemists use moles?

mol1

106.02 23 atoms

Kofatoms9.2641mol1

106.02Kmol2.3

23

atoms


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i) Its fun.

ii) Its impossible to count atoms with your hands.

iii)Numbers of moles are smaller and easier to do math with than big numbers of

atoms and molecules.

6) Convert moles of an atom to grams

a) I need 2.0 moles of copper (Cu) for an experiment. How many grams is that?

b) Atomic mass of Cu = 63.55 g/mol (round to 2 decimals)

c) mol is the abbreviation of Mole I know its only one letter different

chemists!!!

7) Converting grams to moles

a) I have 302 grams of silver (Ag). How many atoms of silver do I have?

b) Step 1: Atomic weight of Ag = 107.8682 g/mol

c) Step 2: Calculate

6.2 Molar Mass and Percent Composition

1) Atomic Mass = mass of one mole of an atom

2) Molar Mass= mass of one mole of a substance

3) Calculate Molar Mass

a) Example: Calculate the Molecular Weight (MW) of RbI2

Step 1: Assume you have 1 mole of this molecule and determine how much each element

weighs from the periodic table.

Rb is atomic # 37. How much does each mole of Rb weigh?

85.47 grams/mol Rb

)figs!sig(2Cu130g

127.01g

mol1

g55.63Cumol0.2

figs)sig(3mol2.80

mol2.79966g107.87

mol1

Agg302


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I is atomic # 53. How much does it weigh?

126.90 g/mol I

Step 2: Determine how many of each element you have

Look at the formula: RbI2

We have 1 Rb atom and 2 I atoms

Step 3: Add all the masses together

You will need to show this work:

Because the units are the same we can add these two numbers together, so

253.80 g/mol + 85.47 g/mol = 339.27 g/mol

339.27 g/mol is the molar mass

4) Converting from moles of a compound to grams

a) Example: I need 3.00 mol NaCl for an experiment. How many grams is that?

Step 1: Find the molar mass

Molar mass = 22.09g/mol + 35.45g/mol

= 57.54 g/mol

Step 2: Use the molar mass like a conversion factor.

5) Converting from grams of a compound to moles

a) Example: How many moles are in 10.0 g of Na2SO4?

Step 1: Find the molar mass.

Molar mass = 142.1 g/mol

g/mol253.80I)(2g/mol)90.126(

g/mol85.47Rb)1(Rb)g/mol47.85(

plus

NaCl173g

g62.172mol1

g54.57mol00.3


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Step 2: Use the molar mass like a conversion factor. You need grams on the

bottom of the fraction.

6) Calculate percent composition

a) Just like any other %

b) Stuff = grams of elements

7) Calculate percent composition

a) Ex: calculate % of Cu and S in Cu2S

Stuff = grams Cu

(63.55 g/mol Cu)(2 mol Cu) = 127.1g Cu

Total stuff = grams Cu + grams S

= 127.1 g + 32.07 g = 159.17 g = 159.2 g

6.3 Formulas of Compounds

1) Identify an empirical formula and a molecular formula

a) Empirical formulasimplest ratio of atoms of each element in a compound

(whole numbers only)!!b) Molecular formulaactual # of atoms of each element in a compound

nCompositioPercent100xstufftotal

stuff

%84.79100xg159.2

g127.1

2

2 4

1 molmoles of Na SO = 10.0 g 7.04 10 mol

142.1 g


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2) Using % composition to determine a formula

a) Law of Definite CompositionAny amount of a pure compound will always

have the same ratio of masses for the elements that make up that compound

b) Ex: H2O is always 88.9% O and 11.1% H by mass

i) Only the simplest formula (ratio) can be found in other words, you can only

find empirical formulas

c) Process is as follows:

i) Calculate % by mass of each element

ii) Determine mass of each element

1. Easy if you use 100 g of the chemical

2. Use mass to find the # of moles of each element

3. Find the smallest ratio of the atoms

4. the number of moles of each element by the element with the smallest # of moles

5. Round to the nearest whole #

d) A molecule is 75% C & 25% H. Calculate the empirical formula.

i) Using 100g total = 75g C and 25 g H

ii) Calculate moles of each =

iii)Ratio = 6.24C : 24.8H, simplify by each by 6.24 Whole number only!!

46.2

25H1

6.2

6.2C

mol251.01g

mol1Hg25mol2.6

12.01g

mol1C75g


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iv)Final ratio 1C : 4H so CH4

v) When you solve this problem, make a table AND show your work:

C H

Percent 75% 25%100 g total 75g 25g

Moles

12.01g

mol1C75g

1.01g

mol1Hg25

6.2 mol 25 mol

Ratio 6.2

6.2=

6.2

25=

1 4

Formula CH4

3) Find the molecular formula

a) Ex: C3H6O2 is an empirical formula for a chemical. The molar mass of the

compound is 148 g/mol.

What is the molecular formula of the compound??

i) Point: The ratio of C:H:O will always be what the empirical formula shows

b) Steps

1. Calculate the empirical formula mass

2. Calculate molar mass/empirical formula mass

3. Multiply your subscripts by that #.

4) Example:

1. Calculate the empirical formula mass:

C3H6O2 mass = (3)(12.01) + (6)(1.01) + (2)(16.00)

= 148.09 g/mol

2. Calculate (molar mass)/(empirical formula mass). Round to a WHOLE number.

3. Multiply the subscripts of the empirical formula by that number.

i) C3x2H6x2O2x2 = C6H12O4

274

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