Molar Mass and the
Mole
- Writing and Balancing Chemical Reactions
- Reactant(s) g
Product(s). Mass does not change during the reaction. You
start and end with the same exact mass.
- The # of atoms does not
change during the reaction. The identity of the atoms does not
change during the reaction. If you start with four carbon atoms,
you must end with four carbon atoms.
- The charge stays the same.
If all the reactants are neutral overall, the products overall must be
neutral. Bonds do change. Bonds break and form during a
chemical reaction.
- Law of Conservation of
Mass =
mass cannot be created or destroyed. This is why we balance
reactions.
- Note that H2O
means there are 2 H atoms and 1 O atoms bonded together in the molecule
while 2 H2O means there are two separate water molecules
- Draw pictures to help
you count atoms. Balancing reactions is largely trial and
error. You put numbers in front of the compounds and check to see
if that helped you get closer to having the same number of atoms on both
sides.
- The first step is to
write the reaction with the correct compounds. Remember that
charges determine the formula of the compound so that the compound is
neutral, such as NaCl, MgCl2,
etc... Once you determine the correct formula you CAN NOT change
the subscripts. If you need two sodium atoms, don't change NaCl to Na2Cl - there is no such
thing.
- We balance chemical
reactions by adding coefficients - numbers in from the the compound. If you need two sodiums, you write 2 NaCl
which means you have 2 distinct and separate NaCl
units.
- Formation Reactions
- In general, element
+ element g
compound
where the elements are in their naturally occurring state (solid, liquid
or gas) and diatomic or not.
- Write and balance the
formation reaction between hydrogen and oxygen: 2 H2(g)
+ O2(g) g 2 H2O
- Write and balance the
formation reaction between sodium and chlorine: 2 Na(s)
+ Cl2(g) g 2 NaCl
- Other examples
- 4 As(s) + 5 O2(g)
g
2 As2O5
- C(s) + O2(g)
g
CO2
- I2(s) + 7
Cl2(g) g 2 ICl7
- 2 Li(s) + Br2(l)
g
2 LiBr
- 4 Fe(s) + 3 O2(g)
g
2 Fe2O3
- Combustion Reactions
- In general for
hydrocarbons, CxHy
+ O2(g) g CO2(g) + H2O(g)
- Write and balance the combustion
reaction for C2H4: C2H4
+ 3 O2(g) g 2 CO2(g) +
2 H2O(g)
- Write and balance the
combustion reaction for C3H8: C3H8
+ 5 O2(g) g 3 CO2(g) +
4 H2O(g)
- Molar Mass (MM) (aka Formula Weight, Molecular Weight)
- Molar Mass is the sum of
all the atomic masses
- Examples: Check
yourself.
- H2O is 18.0
g/mol
- NaCl is 58.5 g/mol
- Ca3(PO4)2
is 310.3 g/mol
- Avogadro's Number - The Mole
- The mole is just a
number like a dozen is 12.
- A mole is a large
number = 6.02 x 1023. This number is Avagadro's
Number and is given the symbol NA
- A mole of something
will not weigh the same as a mole of something else, just like a dozen
eggs do not weight the same as a dozen cars.
- We measure things by
weighing them or counting them. One dozen = 12 anythings
just like one mole = 6.02 x 1023 anythings.
It is convenient to use dozens for cookies, donuts, and eggs. It is
convenient to use moles for atoms and molecules because they are so
small. We cannot actually count out atoms or molecules however.
- one dozen donuts = 12
donuts
- one mole of donuts =
6.02 x 1023 donuts
- one mole of iodine =
6.02 x 1023 atoms of iodine
- one mole of water =
6.02 x 1023 molecules of water
- one mole of marbles =
6.02 x 1023 marbles (which would fill up the Pacific ocean)
- The periodic table
shows us the mass of one mole of atoms for each element. (MM)
- 6.02 x 1023
carbons atoms = one mole = 12.0 grams thus 12.0 g/mol for carbon
- 6.02 x 1023
oxygen atoms = one mole = 16.0 grams thus 16.0 g/mol for oxygen.
- 6.02 x 1023
molecules of O2 masses 32.0 grams. So O2 is
32.0 g/mol
- 6.02 x 1023
water molecules = one mole = 18.0 grams thus 18.0 g/mol for water.
- Practice Questions.
- one mole of Neon has how
many atoms?
answer: 6.02 x 1023
- 23.0 grams of sodium
is how many atoms?
answer: this is one mole so 6.02 x 1023 atoms
- 6.02 x 1023
H atoms weigh how
much?
answer: 1.0 gram
- one mole of carbon
dioxide weighs how
much?
answer: 44.0 grams (add up one C and two O)
- Molar ratios - Use molar
ratios to go between moles of a molecule and moles of its atoms
- The ratio of atoms per
molecule such as (2 H / 1 H2O)
is a molar ratio.
- These ratios can be
used in calculations such as:
- How many moles of H
atoms are in 23.0 moles of NH3?
Answer is 23.0 mol NH3 ( 3 H / 1 NH3 ) =
69.0 mol H atoms
- How many moles of Cl
atoms are in 4.2 moles of
chlorine?
Answer is 4.2 moles Cl2 ( 2 Cl / 1 Cl2 ) =
8.4 mol Cl atoms
- How many moles of
Chloride ions are in 4.4 moles of table salt?
Answer is 4.4 mol NaCl
(1 Cl- / 1 NaCl ) = 4.4 mol Cl- ions
- Examples: Converting
between moles and the number of actual atoms or molecules using NA.
- How many atoms are in
2.50 moles of carbon? Answer
2.50 mol C (6.02 x 1023 atoms/mol) = 1.51 x 1024 C
atoms
- How many moles are in
5.00 x 1023 N
atoms? Answer 5.00 x 1023
N atoms ( mol / 6.02 x 1023 atoms) = 0.831 mol N
- How many molecules are
in 4.7 mol NH3?
Answer 4.7 mol NH3 (6.02 x 1023 molecules/mol) =
2.8 x 1024 molecules NH3
- Consider this balanced
reaction and what it means: 3 H2 (g) + N2
(g) g
2 NH3
- 3 molecules of
hydrogen gas react with one molecule of nitrogen gas to make 2 molecules
of ammonia
- 3 moles of hydrogen
gas react with one mole of nitrogen gas to make 2 moles of ammonia
- 3(2.0 g/mol) + 28.0
g/mol = 2(17.0) g/mol (Each side weighs 34 grams which follows the
Law of Conservation of Mass)
The End
Practice Problems Below
Now we are
ready to practice writing and balancing chemical reactions. Remember,
once you have written the reaction and have the correct formulas, you can NOT
change the subscripts. You can only add coefficients in front of the
compounds to balance the reaction.
- Balance the following formation reaction: Ca (s)
+ I2(s) g CaI2
- Balance the following formation reaction: Na (s)
+ Br2(l) g NaBr
- Write and balance the formation reaction for magnesium
and chlorine.
- Write and balance the formation reaction for potassium
and oxygen.
- Write and balance the formation reaction for calcium
and oxygen.
- Write and balance the formation reaction for phosphorus
pentachloride.
- Balance the following combustion reaction: C4H8
+ O2(g) g CO2(g) + H2O(g)
- Balance the following combustion reaction: C6H14
+ O2(g) g CO2(g) + H2O(g)
- Write and balance the combustion reaction for C6H6.
- Write and balance the combustion reaction for C3H6.
- Balance the following reaction: CaI2
+ Na2SO4 g CaSO4 + NaI
- Balance the following reaction: MgSO4
+ KNO3 g Mg(NO3)2 + K2SO4
Answers
- already balanced
- 2 Na (s) + Br2(l) g 2 NaBr
- Mg (s) + Cl2(g) g MgCl2
- 4 K (s) + O2(g) g 2 K2O
- 2 Ca (s) + O2(g) g 2 CaO
- 2 P(s) + 5 Cl2(g) g 2 PCl5
- C4H8 + 6 O2(g) g 4 CO2(g) + 4
H2O(g)
- 2 C6H14 + 19 O2(g) g 12 CO2(g) +
14 H2O(g)
- 2 C6H6 + 15 O2(g) g 12 CO2(g) +
6 H2O(g)
- 2 C3H6 + 9 O2(g) g 6 CO2(g) + 6
H2O(g)
- CaI2 + Na2SO4 g CaSO4 + 2 NaI
- MgSO4 + 2 KNO3 g Mg(NO3)2
+ K2SO4
Practice Problems
- How many atoms are in 4.00 moles of carbon?
- How many moles are in 1.25 x 1025 atoms of
neon?
- How many molecules are in 2.99 moles of ammonia?
- How many moles are in 5.25 x 1024 units of
lithium sulfide?
- How many units are in 4.24 moles of sodium chloride?
- How many H atoms are in 8.35 moles of water?
- How many O atoms are in 1.44 moles of magnesium
sulfate?
- If I have 3.45 x 1025 H atoms in a pure CH4
sample, how many moles of CH4 are present?
- What is the molar mass of ReH7, C6H6,
KNO3, CuCl2, H2O2, Ne, CaSO4,
Mg3(PO4)2, (NH4)2CO3,
Na?
- 6.02 x 1023 Cl atoms weigh how much?
- One mole of carbon dioxide contains how many
molecules?
- How many moles are in 7.25 x 1026 molecules
oxygen gas?
ANSWERS
- 4.00 mol C (6.02 x 1023 atoms / mol) = 2.41
x 1024 C atoms
- 1.25 x 1025 atoms Ne ( mol / 6.02 x 1023
atoms) = 20.8 mol Ne
- 2.99 mol NH3 (6.02 x 1023
molecules / mol) = 1.80 x 1024 molecules NH3
- 5.25 x 1024 units Li2S (mol /
6.02 x 1023 units) = 8.72 mol Li2S
- 4.24 mol NaCl
(6.02 x 1023 units / mol) = 2.55 x 1024 units NaCl
- 8.35 mol H2O ( 2 H / 1 H2O) (6.02
x 1023 atoms / mol) = 1.01 x 1025 H atoms
- 1.44 mol MgSO4 ( 4 O / MgSO4)
(6.02 x 1023 atoms / mol) = 3.47 x 1024 O atoms
- 3.45 x 1025 H atoms ( 1 CH4
molecule/ 4 H atoms) (mol / 6.02 x 1023 molecules) = 14.3 mol
CH4
- ReH7-193.2 g/mol, C6H6-78.0
g/mol, KNO3-101.1 g/mol, CuCl2-134.5 g/mol, H2O2-34.0
g/mol, Ne-20.2 g/mol, CaSO4-136.2 g/mol, Mg3(PO4)2-262.9
g/mol, (NH4)2CO3-96.0 g/mol,
Na-23.0 g/mol
- 35.5 grams - cause it is one mole - just look at the
Periodic Table
- 6.02 x 1023
- 7.25 x 1026 molecules O2
(mol/6.02 x 1023 molecules) = 1.20 x 103 mol O2