Chapter 3 ­ Stoichiometry
Chapter 3 ­ Stoichiometry
Considering the quantities of materials consumed and produced in chemical reactions is an area of study called chemical stoichiometry.
Atomic mass is the mass of a single atom or isotope of an element in amus
Atomic weight (average atomic mass) refers to a weighted average of all the isotopes of an element. Therefore, atomic weight as reported on the periodic table is most often not a whole number.
Stochiometry Part I
3.2 Atomic Masses
Because Atoms are so tiny, it is imparactical to report their masses in grams, so an arbitray unit called the atomic mass unit (amu)was developed.
1 amu = 1.66 x 10­24 g or 1 g= 6.02 x 1023 amu
Mass Spectrometer
p. 78 in text
A gaseous sample of material is subjected to a stream of high energy electrons. The electrons produce (+) charged ionic particles because the stream knocks electrons off the atoms or molecules being analyzed.
The (+) ions are accelerated down an evacuation tube by a negatively charged grid.
How do you calculate average atomic masses (atomic weights)?
The Ions are shot through a focusing slit to produce a narrow stream of (+) ions.
You need two values, the % abundance of each isotope making up the element and the mass # of each isotope!
The ions are then deflected by a magnetic field.
Example: Cu consists of 69.17% copper­63, which has a mass of 62.929598 amu, and 30.83% copper­65, which has a mass of 64.927793 amu. Calculate copper's average atomic mass.
They then separated by way of mass/charge ratio.
Most ions are 1+, so deflection depends on mass. Lighter ones are deflected more than heavier ones.
The instrument gives precise information on atomic masses, relative abundance of each isotope, and therefore atomic weights.
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Chapter 3 ­ Stoichiometry
(0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu) = 63.55 amu
(copper­63)
+
(copper­65) = the weighted average atomic mass of Cu
Stochiometry Part I
3.3 The Mole
In 1811, Amadeo Avagadro suggested that equal volumes of all gases under equal conditions of temperature and pressure contained the same number of molecules.
Big deal, right? Actually, this may have been the single most important contribution to chemistry and perhaps all science up to that point in man's history. The discovery lead to the concept of the mole, the entire branch of chemistry known to analytical and quatitative chemistry!!
What Is A Mole?
The term mole is classified as a counting number, a number used to specify a certain number of objects.
A mole is 6.0221367 x 1023 objects!!!
Those objects have a mass. And since different objects have different masses, a mole of one type of object has a different mass than a mole of another type of object, in much the same way that a dozen marbles has a different mass than a dozen bowling balls. There are 12 of each item, but their weights and masses are vastly different.
Avogadro's Number (Constant)
• 1 mole of any element or compound (pure substance) contains 6.022 X 1023 particles, molecules/formula units, atoms or ions, depending if we are talking about a element, molecule/compound, or bonded together atoms. This number is Avogadro’s Number or Constant.
• 1 mole of 12C equals 12 grams, so in those 12 grams there are 6.022 X 1023 atoms, • 1 mole of CO2 (molecule) has a mass of 44.00g, so in those 44.00 grams there are 6.022 x 1023 molecules.
• 58.5 g of NaCl cpntains 6.02 x 1023 NaCl formula units = 1 mole of NaCl
• Let's say that real clearly: one mole of ANYTHING contains 6.022 x 1023 entities!!!
Molar Mass and Molecular Weights
• The mass of one mole of a pure substance (element or compound/molecule) is called the molar mass.
• Expressed in g/mol (grams per mole)
• It is numerically = to the average atomic mass of an element or masses added up in a compound/molecule. So, Molar Mass is the average atomic mass or masses with the units of g/mol.
• Example:
Li (element) has a molar mass of 6.94 g/mol (grams (mass) per mole)
• 6.94 g is the mass of 1 mole of Lithium.
• Or, for every 1 mole of Li it has a mass of 6.94g
Conversions ­ Moles of atoms to Grams ­ Gram to Moles of atoms
Using Molar Mass as a Conversion Factor
How many grams are there in 3.6 mol of Ca?
Conversion factor ­ choice of two
1 mole Ca or 40.08g Ca
Write what you are
40.08g Ca
1 mol Ca
Given with unit and
substance!
= 144.29 g Ca
x 40.08g Ca
1 mol Ca
How many moles are there in 7.8g of Si?
Conversion factor ­ choice of two
1 mole Si or 28.09g Si
28.09g Si
1 mol Si
3.6 mol Ca
7.8g Si
1mol Si
28.09 g Si
= 0.28` mol Si
Atoms: The Building Blocks of Matter
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Chapter 3 ­ Stoichiometry
Stochiometry Part I
Calculation of Molar Mass of a Compound
Molar Mass of a Compound/Molecule
What is the mass of one mole of NaCl?
• Na’s average atomic mass is 22.990 g/mol
• Cl’s is 35.453 g/mol
• 35.453+22.990 =58.443 g/mol (grams per mole )
• AS YOU ADD UP THE MASSES, MAKE SURE THAT YOU USE ALL THE NUMBERS YOU SEE IN THE PERIODIC TABLE. THEN, YOU CAN ROUND AND REPORT TO THE HUNDRETH! • 1 mole of NaCl has a mass of 58.44 g/mol
• A molar mass of 58.44 g/mol.
What is the molar mass of Ba(NO3)2?
Ba = 137.327 g/mol
(NO3) = ( N = 14.0067 + 3(15.9994)) = 62.0049 g/mol
2 (62.0049) = 124.0098 g/mol
Ba + (NO3)2
137.327 + 124.0098 = 261.34 g/mol
1 mole of barium nitrate has a mass of 261.34g
Atoms: The Building Blocks of Matter
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Conversions ­ Moles to Gram ­ Gram to Moles
Using Molar Mass as a Conversion Factor
COMPOUNDS/MOLECULES
How many grams are there in 3.6 mol of Ca(OH)2?
Conversion factor ­ choice of two
Write what you are1 mole Ca(OH)2 or 74.09g Ca(OH)2
Given with unit and74.09g Ca(OH)2
1 mol Ca(OH)2
substance!
3.6 mol Ca(OH)2 X 74.09g Ca(OH)2 = 266.73 g Ca(OH)
2
1 mol Ca(OH)2
How many moles are there in 7.8g of SiO2?
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Conversions ­ Moles to Atoms/Particles ­ Vise Versa
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Memorize : 1 mole of any substance contains 6.022 x 10
atoms/particles/molecules/formula units/ions
How many atoms are contained in 3.2 mol of F?
Possible Conversion Factors (1 mole = 6.022 x 1023 atoms)
1 mol of substance or 6.022 x 1023 atoms
6.022 x 1023 atoms
1 mol of substance
3.2 mol F
X 6.022 x 1023 atoms = 1.9 x 1024 atoms
1 mol F
of F
How many moles are in 4.2 x 1024 atoms of C?
7.8g SiO2 X
1mol SiO2
60.08 g SiO2
= 0.13 mol SiO2
= 6.97 mol
4.2 x 1024 atoms C X 1 mol of C
of C
6.022 x 1023 atoms of C
Note: A molar mass conversion factor is not used!!!
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Atoms: The Building Blocks of Matter
Conversions ­ Grams to Molecules/Formula Units ­ Vise Versa
How many mlcs are contained in 3.2 g of HF?
3.2 g F
X 1mol F X 6.022 x 1023 mlcs
20.00g F 1 mol F
= 9.6 x 1022 mlcs
of HF
How many molecules are contained in 5.23 g of KNO3?
5.23 g KNO3X 1mol KNO3 X 6.022 x 1023 mlcs KNO3
101.10g KNO3 1 mol KNO3
= 3.1 x 1022 mlcs
of KNO3
How many grams are in 4.2 x 1024 mlcs of CO?
How many grams are in 4.2 x 1024 mlcs of CO2?
4.2 x 1024 mlcs C X 1 mol of C
X 44.01 g C
6.022 x 1023 mlcs of C 1 mol C
= 307 g
of CO2
Atoms: The Building Blocks of Matter
Conversions ­ Grams to Molecules/Formula Units ­ Vise Versa
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4.2 x 1024 mlcs CO X 1 mol of CO
X 28.01 g CO
6.022 x 1023 mlcs of CO 1 mol CO
= 195.4 g
of CO
Atoms: The Building Blocks of Matter
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Chapter 3 ­ Stoichiometry
Stochiometry Part I
Chemical Formulas give atom ratios as well as mole ratios!
What mass of hydrogen is present in 3.6 mol of H2O?
3.6 mol H2O x
2 mole H x 1.01g H
1mol H2O
= 7.3 g H
1 mol H
Percent Composition from Formulas
First you calculate the molar mass of the compound/
molecule, which represent the whole of your percentage.
Then you take the mass of each individual atom that it contributes to the whole molar mass, and finally divide that by the mass of the compound/molecule, which would represent the part. You multiply by 100 to create the %age then you are done!
Part/Whole x 100 = percent composition Example:
Find the % composition of lead(II) chloride.
First, write the formula: PbCl2
Molar mass = 207.19 g/mol + (2 x 35.45 g/mol) = 278.09
g/mol
Pb = 207.19 g/mol x 100 = 74.50%
278.09 g/mol
Cl = 70.9 g/mol
x 100 = 25.5%
278.09 g/mol
Same Problem using Dimensional Analysis (Your book uses) Find the % composition of lead(II) chloride.
First, write the formula: PbCl2
Mass of Pb = 1 mol Pb x 207.19 g= 207.19 g
1 mol Pb
Mass of Cl = 2 mol Cl x 35.45g = 70.9 g
1 mol Cl
1 mole of PbCl2 = (207.19 + 70.9) = 278.09 g/mol
Pb = 207.19 g/mol x 100 = 74.50%
278.09 g/mol
Cl = 70.9 g/mol
x 100 = 25.5%
278.09 g/mol
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Attachments
AVagadro's Number History