Mass Relationships in Chemical Reactions

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Mass Relationships in Chemical Reactions

• Chapter 3
• 4 - 5 Lectures
• Dr. Ali Bumajdad

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Chapter 3 Topics

• Average Atomic Masses
• The Mole
• Molar Mass (M.m.) and Molecular mass (M.w.)
• Percent Composition
• Calculation Empirical Molecular Formula
• Balancing Chemical Equation
• Stoichiometric Calculations Limiting Reactant
• Theoretical yield, actual yield and percentage
  yield

Dr. Ali Bumajdad
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Atomic mass Average Atomic Masses
Macro World grams Molar mass
Micro World atoms molecules Amu Atomic mass

• Atomic mass is the mass of an atom in atomic mass
  units (amu)

By definition 1 atom 12C weighs 12 amu
On this scale 1H 1.008 amu 16O 16.00 amu
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Natural lithium is 7.42 6Li (6.015 amu) 92.58
7Li (7.016 amu)
Average atomic mass of lithium
6.941 amu
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Sa Ex 3.1 Cu vaporized in a mass spectrometer
and It was found to have two isotopes one of
mass 62.93 amu and abundance 69.09 and the
other of mass 64.93 amu and abundance
30.91 Find its average mass.
Av. m 63.55 amu
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SI unit of amount of substance

• The Mole

• mole amount of a substance that contains
  6.0221367 x 1023 objects.
• mole number of carbon atoms in 12 g of 12C

1 mol NA 6.0221367 x 1023
Avogadros number (NA)
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12.00 grams of 12C contains 6.022 x 1023 1.008
grams of H contains 6.022 x 1023
6.00 grams of 12C contains 3.011 x 1023
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Molar Mass (M.m.) and Molecular mass (M.w.)
eggs
Molar mass is the mass of 1 mole of
in grams
shoes
marbles
atoms
1 mole 12C atoms 6.022 x 1023 atoms 12.00 g 1
12C atom 12.00 amu
1 mole lithium atoms 6.941 g of Li
For any element atomic mass in amu molar mass
in grams/mol
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• From the periodic table

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One Mole of atoms
S
C
12.01 g
32 g
200.59 g
Hg
55.85 g
63.55 g
Cu
Fe
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Molecular mass (or molecular weight) is the sum
of the atomic masses (in amu) in a molecule.
1 molecule SO2 64.07 amu 1 mole SO2 64.07 g
SO2
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For any compound Molecular mass in amu molar
mass in grams/mol
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One Mole of molecules
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Q) 1 mol of apple contains 6.022 x 1023 apple
Q) 1 mol of CH3OH contains 6.022 x 1023 CH3OH
Q) 1 mol of CH3OH contains 6.022 x 1023 O atoms
Q) 1 mol of CH3OH contains 4 x 6.022 x 1023 H
atoms
Q) 1/2 mol of CH3OH contains (6.022 x 1023)/2
CH3OH
Q) 1/2 mol of CH3OH contains (4 x 6.022 x 1023)/2
H atoms
? N n NA
? N no. of dozen 12
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n number of mole
m mass
NA Avogadros number
N number of objects
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No. of A atoms
No. of AaBb molecules
No. of AaBb molecules
No. of A moles

• Q) 1.0 103 CH4 molecules contain
• How many H atoms.
• 2) How many moles of H atoms.

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Q) How many atoms are in 0.551 g of potassium (K)
?
I need n and NA
Find n using
n 0.551 g / 39.10 g mol-1
n 0.0141 mol
N (0.0141) (6.022 x 1023) 8.49 x 1021 K atoms
Expected
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Q) How many H atoms are in 72.5 g of C3H8O ?
Answer 5.82 x 1024 atoms H
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Because
?
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Sa Ex. 3.2 Calculate the mass in gram of 6
atoms of Americium (Am)
Method 1 Using Eq. 3
Method 2
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Sa Ex. 3.3 Calculate the number of moles and
the number of atoms in 10.0 g sample of Al.
Use Eq. 1 and 2
n 0.371 mol Al
N 2.23 1023 Al atoms
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Sa Ex. 3.4 How many Si atoms in 5.68 mg of
silicon computer chip?
N 1.22 1020 Si atoms
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Sa Ex. 3.5 Calculate the number of moles and
the mass in gram of a sample of Co containing
5.00 1020 atoms
Method 1
m of 5.00 1020 atoms 4.89 10-2g
n 8.30 10-4 mol
Method 2
n 8.30 10-4 mol
m of 5.00 1020 atoms 4.89 10-2g
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Sa Ex. 3.6 (1) Calculate the molar mass of
C10H6O3. (2) A sample of 1.56 10-2g
of C10H6O3, how many moles does this sample
represent
(1) M.m. 174.1g (2) No. of moles 8.96 10-5
mol
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Sa Ex. 3.7 (1) Calculate the molar mass of
CaCO3. (2) A sample of 4.86 moles of
CaCO3, what is the mass of the CO32- ions
present?
M.m. 100.09 g/mol
mass of the CO32- 292 g
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Sa Ex. 3.8 Bees release Isopentyl acetate
(C7H14O2) when sting. The amount
release is about 1 ?g (1) How many
molecules of C7H14O2 are released (2)
How many atoms of Carbon are present
(3) How many atoms are present
(1) no. of molecules 51015 molecules (2) no. of
C atoms 4 1016 C atoms (3) no. of atoms 1
1017 atoms
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We should use Eq.1 and Eq. 2 and
Number of H atoms 1.03 1024 H atoms
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We should use Eq.1
Mass of Zn 23.3 gram
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We should use Eq.1 and Eq. 2
Number of S atoms 3.06 1023 S atoms
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• Percent Composition

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A) If I know M.F.
Suppose I have AaBb molecule
52.14 13.13 34.73 100.0
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B) If I do not know M.F. but I know the masses
Suppose I have AaBb molecule
m of C 24 g m of H 8g
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H 3.086
P 31.61 O
65.31
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• Calculation Empirical Molecular Formula

A) Empirical formula from combustion of known
amount

• Find the moles of H and C from mass of H2O and
  CO2
• 2) Find mass of O (or any other element) using
• Mass of O mtotal (mHmC)
• Find the moles of O from mass of O
• 4) Divide by the smallest mole value

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Combust 11.5 g ethanol
22.0 g CO2
13.5 g H2O
0.5 mol C
1.5 mol H
0.25 mol O
Empirical formula C0.5H1.5O0.25
Divide by smallest subscript (0.25)
Empirical formula C2H6O
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Q) 0.1156g f compound contains C, H and N only.
The H2O Absorber increase by 0.1676g and the CO2
absorber increase by 0.1638g what is the
empirical formal of the compound?
The E.F. is CH5N
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B) Empirical Formula from element masses or mass
1) Find the moles of elements (for mass assume
you have 100 g) 2) Divide by the smallest mole
value
C) Molecular Formula from element masses or mass
and M.m.M.F.
1) Find the moles of elements (for mass assume
you have 100 g) 2) Divide by the smallest mole
value. Now you know E.F. 3) Use
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M.F. N2O4 M.m. 90.02 g/mol
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• Sa. Ex. 3.11 Compounds consists of 71.65 Cl,
  24.27C
• And 4.07 H and its molar mass 98.96 g/mol
• Determine its E.F.
• Determine its M.F.

(Assume that we have 100 grams of compound)

1. E.F. CH2Cl
2. M.F. C2H4Cl2

Dr. Ali Bumajdad
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• Sa. Ex 3.12 White powder contain 43.64 P and
  56.36 O. The compound molar mass 283.88 g/mol.
• What is E.F.
• What is M.F.

1) Find the moles of elements (for mass assume
you have 100 g)
2) Divide by the smallest mole value
E.F. P2O5 M.F. P4O10
Dr. Ali Bumajdad
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Balancing Chemical Equation
Reactant
Product
2 atoms Mg 1 molecule O2 makes 2 formula units
MgO 2 moles Mg 1 mole O2 makes 2 moles MgO 1
moles Mg 1/2 mole O2 makes 1 moles MgO 48.6
grams Mg 32.0 grams O2 makes 80.6 g MgO
NOT
2 grams Mg 1 gram O2 makes 2 g MgO
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Balancing Chemical Equations
1.Start by balancing those elements that appear
in only one reactant and one product.
start with C or H but not O
2. Balance those elements that appear in two or
more reactants or products.
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Dr. Ali Bumajdad
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Stoichiometric Calculations Limiting Reactant
Calculation involving masses
( Coefficient provide moles ratios not exact
amount)

1. Balanced the equation
2. Find moles of one reactant or product
3. Use coefficient ratio to calculate the moles of
   the required substance
4. Convert moles of required substance into masses

Dr. Ali Bumajdad
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Q) How many moles of C atoms are needed to
combined with 4.87 mol Cl to form C2Cl6
1) Balanced the equation
2) Find moles of one reactant or product
3) Use coefficient ratio to calculate the moles
of the required substance
Dr. Ali Bumajdad
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Q) 209 g of methanol burns in air according to
the equation Find mass of water formed.
1) Balanced the equation
2) Find moles of one reactant or product
3) Use coefficient ratio to calculate the moles
of the required substance
4) Convert moles of required substance into masses
m H2O n M.m.13.0418.016 235 g H2O
Dr. Ali Bumajdad
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Q) How many grams of Ca must react with 83.0 g of
Cl2 to form CaCl2
1) Balanced the equation
2) Find moles of one reactant or product
3) Use coefficient ratio to calculate the moles
of the required substance
4) Convert moles of required substance into masses
m Ca n M.m. 1.1740.08 46.9 g Ca
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1) Balanced the equation
2LiOH(s) CO2 (g) Li2CO3(s)
H2O(L)
2) Find moles of one reactant or product
3) Use coefficient ratio to calculate the moles
of the required substance
4) Convert moles of required substance into masses
m CO2 n M.m. 20.8844.0 920 g CO2
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(the reactant the completely consumed in a
chemical reaction)
Limiting Reactant
6 green used up
6 red left over
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2 mol 1mol 2mol
1 mol 1/2mol 1mol
2.1 mol 1mol 2mol
limiting
1.9 mol 1mol 1.9mol
limiting
? Limiting reactant determine the amount of
product
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• Why limiting reactant is important?

Because in the stoichiometric calculation I
should only use the coefficient of the limiting
reactant

• How do I know that a reaction contains a
  limiting reactant ?

If I've been given the masses or number of moles
of two reactant then I might have limiting
reactant
Dr. Ali Bumajdad
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Calculation involving a limiting reactant

1. Balanced the equation
2. Find moles of the two reactants
3. Identify the limiting reactant (How?)
4. Use the coefficient ratio between the limiting
   reactant and the required substance to find out
   the number of mole of the required substance
5. Convert moles of required substance into masses

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1) Balanced the equation
2) Find moles of the two reactants
limiting
3) Identify the limiting reactant (How?)
limiting
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3) Use the coefficient ratio between the limiting
reactant and the required substance to find out
the number of mole of the required substance
2.30 mol Al2O3
4) Convert moles of required substance into masses
m Al2O3 n M.m. 2.30101.96 235 g Al2O3
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1) Balanced the equation
2) Find moles of the two reactants
limiting
3) Identify the limiting reactant (How?)
limiting
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3) Use the coefficient ratio between the limiting
reactant and the required substance to find out
the number of mole of the required substance
0.380 mol N2
4) Convert moles of required substance into masses
m N2 n M.m. 0.38028.0 10.6 g N2
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• Theoretical yield, actual yield and percentage
  yield

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1) Balanced the equation
2) Find moles of the two reactants
limiting
3) Identify the limiting reactant (How?)
limiting
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3) Use the coefficient ratio between the limiting
reactant and the required substance to find out
the number of mole of the required substance
2135 mol CH3OH
4) Convert moles of required substance into masses
m CH3OH n M.m. 213532.04 6.86104 g CH3OH
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