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Chemical Accounting
The Chemical Formula
Information that can be derived from a chemical formula:- Elements present- Proportions of elements contained (number of atoms contained in the molecule)- Molecular Weight- Review: Mass number vs. Atomic Mass- Carbon – 12 amu or 12.011 amu(C-12 98.9%, C-13 1.10% in nature)
We measure ordinary objects either by counting or weighing them,
depending on which method is more convenient
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.http://weyume.com/wp-content/uploads/2011/04/rice.jpg
http://farm1.static.flickr.com/21/90994367_5613e69fd9.jpg
Dozen = 12
Pair = 2
Certain nouns can be used to define a collection of objects
The mole
The mole (n or mol) is the amount of matter that contains as many entities (atoms, molecules, ions, or other particles) as there are atoms in
exactly 12 g of the carbon-12 isotope (12C)
• The actual number of atoms in 12 g of carbon-12 was determined experimentally
• Avogadro’s number (NA)
NA = 6.02 x 1023
Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. 8th ed. Phils: Pearson Education Asia Pte. Ltd. Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Just as 1 dozen of oranges contains 12 oranges, 1 mole of matter contains 6.02 x 1023 entities
Brown, , E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. 8th ed. Phils: Pearson Education Asia Pte. Ltd. Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
1 mole 12C atoms = 6.02 x 1023 12C atoms
1 mole H2O molecules = 6.02 x 1023 H2O molecules
1 mole NO3- ions = 6.02 x 1023 NO3
- ions
Each of these contains one mole of the substance
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
copper iron
carbon sulfur
mercury
One mole (or an Avogadro’s number)is an extremely big number
• One mole of softdrink cans would cover the surface of the earth to a depth of over 300 kilometers
• If we were able to count the number of atoms at a rate of 10 million per second, it would take about 2 billion years to count a mole of atoms
Molar mass
The molar mass (M) of a substance is the mass of one mole of its entities (atoms, molecules,
ions, or other particles) in units of g/mol
MH2O = 18.0 g/mol
(one mole of H2O molecule weighs 18.0 g)
MNO3- = 62.0 g/mol
(one mole of NO3- ion weighs 62.0 g)
MC = 12.01 g/mol
(one mole of C atom weighs 12.01 g)
Brown, T., E. LeMay, and B. Bursten. 2000. Chemistry: The Central Science. 8th ed. Phils: Pearson Education Asia Pte. Ltd. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-
Hill.
The periodic table is indispensable for calculating the molar mass of a substance
• Elements
– M is the numerical value from the periodic table
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
MH = 1.008 g/mol
MO = 16.00 g/mol
The periodic table is indispensable for calculating the molar mass of a substance
• Compounds
– M is the sum of the molar masses of the atoms of the elements in the formula
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
MSO2 = MS + (2 x MO)
= 32.07 g/mol + (2 x 16.00 g/mol)
= 64.07 g/mol
The periodic table is indispensable for calculating the molar mass of a substance
• Compounds
– M is the sum of the molar masses of the atoms of the elements in the formula
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
MK2S = (2 x MK) + MS
= (2 x 39.10 g/mol) + 32.07 g/mol
= 110.27 g/mol
Interconverting moles, mass,and chemical entities
(atoms, molecules, ions, or other particles)
The factor-label method is used to convert from one unit to another
1 peso = 4 25-centavos
1 peso
4 25-centavos= 1
1 peso
4 25-centavos= 1
unit factor
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Alexa bought 18 fresh chicken’s eggs. How many dozens of egg did she buy?
dozens of egg = x18 eggs 1 dozen egg
12 eggs
= 1.5 dozens of egg
unit factor
In order to convert between moles, mass, and chemical entities, the factor label method is used
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
Methane (CH4) is the principal component of natural gas. How many moles of methane
are present in 6.07 g of CH4?
MCH4 = MC + (4 x MH)
= 12.01 g/mol + (4 x 1.01 g/mol)
= 16.05 g/mol
Report final answer with the correct number of significant figures!
nCH4 = x6.07 g CH4 1 mol CH4
16.05 g CH4
= 0.378 mol CH4
How many molecules of methane are present 6.07 g of CH4?
molecules CH4 = x 6.07 g CH4 1 mol CH4
16.05 g CH4
= 2.28 x 1023 molecules CH4
x6.02 x 1023 molecules CH4
mol CH4
Glucose (C6H12O6), also known as blood sugar, is used by the body as energy source.
How many moles of glucose are present in 1.75 x 1022 molecules of glucose?
nC6H12O6 = x 1.75 x 1022 molecules C6H12O6 1 mol C6H12O6
6.02 x 1023 molecules C6H12O6
= 0.0291 mol C6H12O6
How many grams of glucose are present in 1.75 x 1022 molecules of glucose?
MC6H12O6 = (6 x MC) + (12 x MH) + (6 x MO)
= (6 x 12.01 g/mol) + (12 x 1.01 g/mol)
+ (6 x 16.00 g/mol)
= 180.18 g/mol
How many grams of glucose are present in 1.75 x 1022 molecules of glucose?
nC6H12O6 = x 1.75 x 1022 molecules C6H12O6 1 mol C6H12O6
6.02 x 1023 molecules C6H12O6
180.18 g C6H12O6x
1 mol C6H12O6
= 5.24 g C6H12O6
1. Urea [(NH2)2CO] is used in animal feeds, as fertilizer and in the manufacture of polymers.
a. Draw the Lewis structure of area. C is surrounded by O and the N’s. (Where are the H’s connected?)
b. Calculate its molar mass.
c. Consider 25.6 g of urea. How many moles of urea present?
d. How many moles of N are present?
e. How many moles of C are present?
f. How many molecules of urea are present?
g. How many atoms of N are present?
2. Vitamin C, ascorbic acid, is often sold as sodium ascorbate.
a. Calculate its molar mass.
b. Consider a 500.-mg tablet. How many moles of sodium ascorbate are present?
c. How many moles of C are present?
d. How many moles of Na are present?
e. How many formula units of sodium ascorbate are present?
f. How many atoms of Na are present?
OO
OH
-O
HOOH
Na+
1. Monosodium glutamate (MSG), a food flavor enhancer, has been blamed for “Chinese Restaurant Syndrome”. Its molecular formula is C5H8NO4Na.
a. Calculate the molar mass of MSG.
b. Consider a 2.00-g sachet. How many moles of MSG are present?
c. How many moles of C are present?
d. How many moles of N are present?
e. How many formula units of MSG are present?
f. How many atoms of O are present?
HO O-
O O
NH2
Na+
Chemical reactions and chemical equations
A chemical reaction shows the process in which a substance (or substances) is
changed into one or more new substances
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
A chemical equation uses chemical symbols to show what happens during a chemical reaction
reactants product
(g) (g) (l)
“Two molecules of hydrogen react with one molecule of oxygen to yield two moles of water”
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
The Law of Conservation of Mass states that matter is neither created nor destroyed
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
To conform with the Law of Conservation of Mass, there must be the same number of each type of atom on both sides of the arrow. Hence, we balance the equation by
adding coefficients before each chemical symbol
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill.
Calculating the amounts of reactant and product
Stoichiometry a double cheeseburger
2 bun slices + 2 cheese slices + 2 burger patties =
In a balanced equation, the number of moles of one substance is equivalent to the number of
moles of any of the other substances
2CO(g) + O2(g) 2CO2(g)
2 mol CO = 1 mol O2
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
2 mol CO
1 mol O2
= 11 mol O2
2 mol CO= 1
In a balanced equation, the number of moles of one substance is equivalent to the number of
moles of any of the other substances
2CO(g) + O2(g) 2CO2(g)
2 mol CO = 2 mol CO2
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
2 mol CO
2 mol CO2
= 12 mol CO2
2 mol CO= 1
In a balanced equation, the number of moles of one substance is equivalent to the number of
moles of any of the other substances
2CO(g) + O2(g) 2CO2(g)
1 mol O2 = 2 mol CO2
Chang, R. 2002. Chemistry 7th ed. Singapore: McGraw-Hill. Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
1 mol O2
2 mol CO2
= 12 mol CO2
1 mol O2
= 1
The amount of one substance in a reaction is related to that of any other
Silberberg, M. 2010. Principles of General Chemistry. 2nd ed. New York: McGraw-Hill.
All alkali metals react with water to produce hydrogen gas and the corresponding
alkali metal hydroxide
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
How many moles of H2 will be formed by the complete reaction of 6.23 moles of Li with water?
nH2 = x6.23 mol Li 1 mol H2
2 mol Li
= 3.12 mol H2
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
How many grams of H2 will be formed by the complete reaction of 80.57 g of Li with water?
2Li(s) + 2H2O(l) 2LiOH(aq) + H2(g)
mH2 = x x80.57 g Li 1 mol Li 1 mol H2
6.941 g Li 2 mol Li
2.016 g H2x
1 mol H2
= 11.70 g H2
In a lifetime, the average American uses about 794 kg of copper in coins, plumbing, and wiring. Copper is obtained from sulfide ores (such as Cu2S) by a multistep process. After an initial
grinding, the first step is to “roast” the ore (heat it strongly with O2) to form Cu2O and SO2
2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)
How many moles of oxygen are required to roast 10.0 mol of Cu2S?
2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)
nO2 = x10.0 mol Cu2S 3 mol O2
2 mol Cu2S
= 15.0 mol O2
How many grams of SO2 are formed when 10.0 mol of Cu2S is roasted?
2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)
mSO2 = x x 10.0 mol Cu2S 2 mol SO2 64.07 g SO2
2 mol Cu2S 1 mol SO2
= 641 g SO2
How many grams of O2 are required to form 2.86 kg of Cu2O?
2Cu2S(s) + 3O2(g) 2Cu2O(s) + 2SO2(g)
mO2 = x x2.86 kg Cu2O 1000 g Cu2O 1 mol Cu2O
1 kg Cu2O 143.10 g Cu2O
3 mol O2 32.00 g O2x x
2 mol Cu2O 1 mol O2
= 960 g O2
Within the cylinders of a car’s engine, the hydrocarbon octane (C8H18), one of many components of gasoline, mixes with oxygen from the air and burns to form carbon dioxide and water vapor.
a.How much carbon dioxide ( in kg) is produced when 2.00 kg of octane is burned ?
b.How much oxygen(in kg) is required to burn the same amount of octane?
2C8H18(l) + 25O2 (g) 16CO2 (g) + 18H2O (g)
Limiting Reactants
The reactant used up first in a chemical reaction is called the limiting reactant. Excess reactants are present in quantities greater than necessary to react with the quantity of the limiting reactant.
A + B --- C + D
Given the amounts of A and B, which is the limiting reactant? How much C and D are produced?
Urea is prepared by reacting ammonia with carbon dioxide:
2NH3(g) + CO2(g) -- (NH2)2CO(aq) + H2O(l)
In one process, 637.2 g of NH3 are allowed to react with 1142 g of CO2.
• Which is the limiting reactant?
• How much urea (in grams) is produced?
• How much of the excess reactant (in grams) is left at the end of the reaction?
Strategy
• Check if the equation is balanced.
• Convert mass of each reactant to moles.
• Calculate the amount of product formed from the each of the reactants.
• The reactant the produces the less amount is the limiting reactant.
1. The reaction between aluminum and iron (III) oxide can generate temperatures around 3000⁰C and is used in welding metals:
Al + Fe2O3 -- Al2O3 + 2Fe
In one process, 124 g of Al are reacted with 601 g of ferric oxide.
1. Which is the limiting reactant?
2. How much Al2O3 (in grams) is produced?
3. How much of the excess reactant (in grams) is left at the end of the reaction?
2. Titanium is a strong & light metal used in rockets & aircrafts. It is prepared by the reaction between titanium (IV) chloride with molten magnesium at around 1000⁰C:
TiCl4 + Mg -- Ti + 2MgCl2
In a certain industrial operation, 3.54 x 107g of TiCl4 are reacted with 1.13 x 107 g of magnesium.•Which is the limiting reactant?•How much Ti (in grams) is produced?•How much of the excess reactant (in grams) is left at the end of the reaction?
COMPOSITION STOICHIOMETRY:
Mass Percentage / % Composition of Elements in a
Compound
What is the percent composition of carbon in the following:a. CO2
b. C6H12O6
Percent Composition
1. Compute for the % composition of the elements in H2SO4.2. Determine the formula for a compound that consists of 40% C, 6.72% H and 53.29% O. Hint: Assume 100g of the compound
Elemental Composition
Empirical formula: Simplest formula for a compound (usually computed through elemental composition)
Molecular formula: Represents the actual number of atoms of each element present in a molecule of the compound
Empirical vs. Molecular Formulae
The elemental composition of a compound was found to be 4.8% H, 57.1% C and 38.0% S. Determine the molecular formula if the molecular weight of the actual compound is 168 g/mol.
Molecular Formula Sample Problem
© 2010 Pearson Prentice Hall, Inc.
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The Mole In Gases
© 2010 Pearson Prentice Hall, Inc.
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The Mole
Molar volume of a gas: One mole of any gas occupies a volume of 22.4 L at standard temperature and pressure (STP).
STP is defined as 1 atmosphere (atm) of pressure and a temperature of 0 oC.
© 2010 Pearson Prentice Hall, Inc.
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The Mole
SOLUTION STOICHIOMETRY
© 2010 Pearson Prentice Hall, Inc.
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Solutions and Concentration
The amount of solute in a given amount of solvent is defined as solution concentration.
A dilute solution contains relative small amounts of solute in a given amount of solvent.
A concentrated solution contains relatively large amounts of solute in a given amount of solvent.
© 2010 Pearson Prentice Hall, Inc.
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Solutions
Molarity (M) is defined as the moles of solute per liter of solution.
M =liter
mol
© 2010 Pearson Prentice Hall, Inc.
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Solutions
Percent Concentration
Percent by volume = x 100
© 2010 Pearson Prentice Hall, Inc.
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Solutions
Percent Concentration
Percent by mass = x 100
Sample Problems• How many moles of nitric acid is present in 10.0
mL of a 1.25 M solution?
• Suppose you have 2.00 mole of sucrose (approx. 685 grams) diluted in 1.75 L of water. What is the concentration of your sugar solution? What is its concentration in percent by mass? (Assume density of water is 1.00 g/mL)
• What is the molarity of a solution that consists of 2.00 grams of KMnO4 (MW: 158.0 g/mol) dissolved into 100.0 mL of solution?