Transcript Molar Mass - Madison County Schools

Bell Ringer

What is a Mole?

What is the mass of a NaCl molecule?
Chapter 7.1b
Common counting units:
shoes by the pair (pair = 2)
eggs by the dozen (dozen = 12)
paper by the ream (ream = 500 sheets).
Counting With Moles
o Chemical reactions usually involve very large
numbers of tiny particles
o We use a counting unit called the mole to
measure amounts of a substance.
o Mole (mol) = amount of any substance that
contains approximately 6.02 × 1023 particles.
o This amount is known as Avogadro’s
number.
o Ex: 1 mole of iron = 6.02 × 1023 atoms of
iron.
Molar Mass:
mass
of one mole of a substance
same as atomic mass, in grams.
Examples:
 Carbon:
Atomic mass = 12.0 amu
Molar mass = 12.0 grams.

Sulfur:
Atomic mass = 32.1 amu
Molar mass = 32.1 grams.
Counting With Moles
o
Molar Mass of a compound =
o sum of atomic masses of all its atoms,
expressed in grams.
o Tells us the total grams in one mole of the
compound
Example: Carbon dioxide (CO2)
• 1 carbon atom = 12.0 amu
• 2 oxygen atoms = (2 × 16.0 amu) = 32.0 amu
• CO2 atomic mass = (12.0 + 32.0) = 44.0 amu
• CO2 molar mass = 44.0 grams
• 44.0g CO2 in 1 mol CO2
Mole-Mass Conversions

Knowing the molar mass of a substance lets you
easily convert between moles and mass
 “converting” means you can calculate one from
the other
Example:
Molar mass of CO2 is 44.0 grams
1.0 mole of CO2 = 44.0 grams
Conversion
factors 
OR
Converting Example: Chocolate
Cake Recipe
Recipe calls for 4 eggs per cake and you plan to make
3 cakes. Eggs can only be bought by the dozen (i.e.
groups of 12). How may dozen eggs do you need to
buy?
Conversion factors: 4 eggs 1 cake
1 cake 4 eggs
12 eggs
1 doz. eggs
1 doz. eggs 12 eggs
Calc: 3 cakes x 4 eggs x 1 doz eggs = 1 doz eggs
1 cake 12 eggs
Counting With Moles
o
o
You have 55.0 grams of CO2. How many
moles?
o To calculate how many moles of CO2 you
have, multiply the mass by a conversion
factor.
You can check your answer.
Chemical Calculations
o In chemical reactions, the mass of a
reactant or product can be
calculated.
o Use a balanced chemical equation
and the molar masses of the
reactants and products.
Chemical Calculations
o
In a balanced chemical equation, the
number of atoms of each element on the
left equals the number of atoms of each
element on the right.
Chemical Calculations
o
The Coefficient tells us the # of moles of
each reactant and product involved the
reaction
Chemical Calculations
o
Molar Mass = the # of grams per mole of
each reactant and product
Chemical Calculations
o
Mass (of a reactant or product) =
o the # of moles in the balanced equation
(Coefficient) x the molar mass.
Chemical Calculations - Stoichiometry
Converting Mass to Moles
To calculate how much oxygen is required to
make 144 grams of water, begin with a balanced
chemical equation for the reaction.
2H2 + O2  2H2O
• Determine how many moles of water you are
trying to make.
• Convert the given mass (144g) of water into moles.
Chemical Calculations
Using Mole Ratios
Write the conversion factors, or mole ratios.
 2H2 + O2  2H2O
Calculate
how many moles of oxygen are
required to produce eight moles of water.
Chemical Calculations
Converting Moles to Mass
Convert moles of O2 to grams of O2 by
using the molar mass of O2 as a
conversion factor.
To
produce 144 grams of H2O, you must
supply 128 grams of O2.
Assessment Questions
1.
Which of the following is a balanced
chemical equation for the reaction of
aluminum and ammonium perchlorate?
a. Al + NH4ClO4 —› Al2O3 + NH4Cl
b. 4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
c. 8Al + NH4ClO4 —› 4Al2O3 + NH4Cl
d. 8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
Assessment Questions
1.
Which of the following is a balanced
chemical equation for the reaction of
aluminum and ammonium perchlorate?
a. Al + NH4ClO4 —› Al2O3 + NH4Cl
b. 4Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
c. 8Al + NH4ClO4 —› 4Al2O3 + NH4Cl
d. 8Al + 3NH4ClO4 —› 4Al2O3 + 3NH4Cl
ANS:
D
Assessment Questions
2.
How many moles of NaCl (molar mass
= 58.5) are there in a 5.85-gram
sample?
a.
b.
c.
d.
58.5 mol
5.85 mol
0.10 mol
0.28 mol
Assessment Questions
2.
How many moles of NaCl (molar mass
= 58.5) are there in a 5.85-gram
sample?
a.
b.
c.
d.
58.5 mol
5.85 mol
0.10 mol
0.28 mol
ANS: C
Assessment Questions
3.
In the reaction shown below, how many
grams of hydrogen are produced when 10
moles of potassium react with water?
2K + 2H2O —› 2KOH + H2
a. 2.5 g
b. 5 g
c. 10 g
d. 20 g
Assessment Questions
3.
In the reaction shown below, how many
grams of hydrogen are produced when 10
moles of potassium react with water?
2K + 2H2O —› 2KOH + H2
a. 2.5 g
b. 5 g
c. 10 g
d. 20 g
ANS: C
Assessment Questions
1.
In the chemical equation below, the
reactants are carbon dioxide and water.
CH4 + 2O2 —› CO2 +2H2O
True <or> False?
ANS: False, products