UNIT 2

Moles and Molar Mass



The Mole represents a specific amount of any substance.
Specifically it represents 6.02x10
23 particles such as atoms and molecules.



The mole is based on the fact that 12 grams of Carbon-12 has a mole of atoms.
A mole means you have 6.02x10
23 particles.



Molar Mass (aka gram formula mass or molecular mass) is the mass of one mole of a substance.
Element ’ s molar masses are reported on the periodic table.

1.
What is the molar mass of iron?
55.8 g/mol
2.
What is the molar mass of copper?
63.5 g/mol

1. What is the molar mass of water?
H2O = 2(1.0) + 16.0 =
18.0g/mol
2. What is the gram-formula-mass of calcium chloride?
CaCl2 = 40.1 + 2(35.5) =
111.1 g/mol

Calculating Moles

Use the formula on table T: number of moles = given mass (g) _ gram-formula mass
(Given mass will be your answer.)

1.
Calculate the mass of 6.70 moles of carbon.
6.70 = x _
12.og/mol x = 80.4 g

Calculate the mass of 0.023moles of lithium.
0.023 = x _
6.9 g/mol x = 0.16 g

Calculate the mass of 25.02 moles of calcium phosphate.
25.02 = x _
310.3/mol x = 7764 g

 Use the same formula to calculate the moles, placing the number in the question on the numerator: number of moles = given mass (g) gram-formula mass

4. Determine the number of moles in 8.0 grams of Boron.
x = 8.0 g _
10.8g/mol x = 0.74 moles

Determine the number of moles in 0.567 grams of Helium.
x = 0.567 g _
4.00 g/mol x = 0.142 moles

Find the number of moles in 1230 grams of magnesium sulfate.
x = 1230 g _
120.4 g/mol x = 10.2 moles

Moles to Coefficients

Mix + 2 Eggs + 1 cup water = Cake
Reactants Products
2 Mix = 2 Cakes
6 Eggs = 3 Cakes

2

2
reactants products
Coefficients: How many moles of the substance are needed in a reaction.

 To relate moles of one substance to another, simply create a proportion:
4Al + 3O
2
 2Al
2
O3
1.
2.
3.
4.
If 3 moles of oxygen react, how many moles of
Aluminum oxide form? 2
If 4 moles of aluminum react, how many moles of oxygen are needed? 3
If 4 moles of aluminum oxide are formed, how many moles of oxygen were used?
6
If 8 moles of aluminum react, how many moles of oxygen are needed? 6

16 Al + 3S
8
 8Al
2
S
3
1.
If 2.50 moles of sulfur react, how many moles of aluminum sulfide form?
2.50 mole S = x moles Al
2
S
3
3 8 x = 6.67 moles

16 Al + 3S
8
 8Al
2
S
3
If 3.75 moles of aluminum react, how many moles of sulfur are needed?
3.75 mole Al = x moles S
8
16 3 x = 0.703 moles

Why do chemists use moles to measure substances? Why aren ’ t grams, liters and molecules enough?

Balancing



In a reaction, atoms and molecules cannot appear or disappear. Mass must stay constant from the beginning to the end of the reaction.
H2 + O2  H2O
 ___H2 + ___O2  ___ H2O

___ N2 + ____H2  ____ NH3
___Li + ____O2  ___Li2O

__Pb(NO 3 ) 2 +__K 2 CrO 4
 ___PbCr 2 O4 + ___KNO 3
___C4H8 + ___O2  ___CO2 + ___H2O

1.
2.
3.
4.
5.
Synthesis:
Decomposition:
Combustion:
A + 2B  AB
2
AB
2
 A + 2B
CH
4
+ O
2
 CO
2
+ H
2
Single Replacement: AB + C  CB +A
O
Double Replacement: AB + CD  AD + CB
* Notice synthesis and decomposition are opposites. Also, combustion can have any carbon compound as a reactant.

Empirical and Molecular Formulas

 Empirical formula refers to any molecular formula in it ’ s reduced form.
Are these empirical? If not, reduce them:
1.
C
2
H
2
2.
C
6
H
12
O
6
3.
NO
2
4.
Na
2
(OH)
2
1. CH
2. CH
2
O
3. NO
2
4. NaOH

 Molecular Formulas are some multiple of the empirical formula.

 If the empirical formula is CH
4 could be CH
4
, C
2
H
8
, C
3
H
12 etc. a molecular formula
A compound whose empirical formula is NH
3 has a mass of 34 g/mol. What is the empirical formula?
NH
3
2NH
3
= 17.0 g/mol 34/17 = 2
= N
2
H
6





Find the mass of the empirical formula.
Divide the mass given by the empirical mass.
Distribute your answer through the empirical formula.
If a compound has a mass of 45 g/mol and an empirical formula of CH
3
, what is the molecular formula?
CH
3
3CH
3
= 15 g/mol 45/15 = 3
= C
3
H
9

Percent Composition



Nutrition Facts on foods can tell you just how much of a substance you are consuming and how that relates to how much you should eat in a day.
It is equally important to know how much of a element or compound is in a mixture.

% By mass: mass part x 100 total mass
1.
Find the % by mass of phosphoric acid.
H3PO4 3(1.0) x 100
3(1.o)+32.1+4(16.0)
= 3.02%

Find the percent by mass of Ca in Ca(OH)
2
.
40.0
40.0+2(16.0)+2(1.0) x 100
= 54.1%

1.
2.
3.
23.0/74.5 *100 =
23.0/58.5 *100 =
30.9%
39.3%
23.0/40.0 *100 = 57.5%

A sample of a substance containing only magnesium and chlorine was tested in the laboratory and found to be composed of
74.5% chlorine by mass. If the total mass of the sample was 190.2 grams, what is the mass of the magnesium?
0.745 (190.2) = 141.7 grams

For teacher use 

I went apple picking and they charge per dozen apples. I was worried it would take forever to count all the apples when I was on line. But the were smart: they weighed the apples. A dozen apples weighs 180 grams.
 I have 540 grams of apples, how many dozen do I
 have?
How much does 2 dozen weigh?


How much does 0.25 dozen weigh?
How is this similar for moles?

1.
Find the gram formula mass of: a.
Ne c. NH
3
2.
3.
4.
b.
NaCl d. Mg(OH)
2
Find the number of moles is 230g Mg(OH)
2
Find the mass of 2.50 moles of NH
3
Given: 2NaCl + Mg(OH)
2
 2NaOH + MgCl
2
Find the number of moles of NaCl that will react with 3.75 moles of Mg(OH)
2
.