Chapter 3 notes ­ solutions.notebook
April 01, 2015
3.1 Percent Composition
The percentage composition of a compound refers to the relative mass of each element in a compound.
The Law of Definite Proportions states that the elements in a given chemical compound are always present in the same proportion by mass.
Water
Hydrogen Peroxide
Oct 8­10:35 PM
The mass of an element in a compound can be expressed as mass percent or PERCENTAGE COMPOSITION (the relative mass of each element in a compound expressed as percent).
Calculating % Composition (2 types):
1. Given Compound Only
2. Given a Mass Sample
{Bubble gum activity}
Sep 30­7:43 PM
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Example 1: A sample of a compound has a total mass of 48.72g. The sample is found to contain 32.69g of zinc and 16.03g of sulfur. What is the percent composition of the compound? Need to take the mass of zinc and divide it by the total mass Need to take the mass of sulfur and divide it by the total mass
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Example 2: A chemical has a total mass of 156.87g and is made up of C, O, and N. The sample contains 83.25g of carbon and 36.55g of O. Calculate the percent composition of each element.
*Don't worry about sig figs...just put the percents to two decimal places!
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Practice Problems
page 82 #1­4
Oct 9­12:19 PM
Practice Problems
page 82 #1­4
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Calculating % Composition from a Formula
Example 1: Determine the percent composition of aluminum hydroxide, Al(OH)3.
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Example 2:
a) Calculate the percent composition of iron (III) oxide.
b) Calculate the mass of iron that could be extracted from a 2500.0g sample of iron (III) oxide. Practice Problems
page 85 #5­8
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Oct 11­9:37 AM
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
3.2 Empirical Formula
The formula of a compound expressed as the smallest possible whole number ratio of subscripts of the elements in the formula.
Molecular Formula Empirical Formula
C2H4
C6H12O6
H2O
C2H6
H2O2
C3H4
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Can different compounds have the same
empirical formula?
Example:
Benzene Ethyne
C6H6
C2H2
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Note: Ionic compounds DO NOT have molecular formulas.
*They exist as a ratio of ions.
*The empirical formula of an ionic compound is usually the same as its chemical formula.
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Steps to Finding Empirical Formula
Percent to mass
Mass to mole
Divide by small
Multiply til'whole
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Example 1: Calculate the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen.
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What would you multiply by to get a whole number?
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Example 2: Determine the empirical formula of a compound that has 80.2% carbon, 9.62% hydrogen and 10.2% oxygen.
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Example 3: Calculate the empirical formula of a compound containing 11.2g of iron and 21.3g of chlorine.
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Chapter 3 notes ­ solutions.notebook
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Example 4: Calculate the empirical formula of a compound that is 85.6% carbon and 14.4% hydrogen.
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Practice Problems
page 89 #9­12
page 91 #13­16
page 94 #2,4,6,7
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Exit Pass
What is the empirical formula of a compound that contains 67.6% mercury, 10.8% sulfur, and 21.6% oxygen?
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Determining the Empirical Formula of a Hydrate
*Many ionic compounds crystallize from a water solution with water molecules incorporated into their structure. This is called a hydrate.
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Every hydrate has a specific number of water molecules bonded to each formula unit.
MgSO4 7H2O
CaSO4 2H2O
Ba(OH)2 8H2O
CaCl2 6H2O
Fe(OH)2 3H2O
*You need to
remember the
prefixes!
Oct 21­7:57 AM
Calculations
Example 1: A 50.0g sample of a hydrate of barium hydroxide, Ba(OH)2, contains 27.2g of Ba(OH)2. Find the value of x in Ba(OH)2 xH2O.
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Chapter 3 notes ­ solutions.notebook
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Example 2: A 3.34g sample of a hydrate has the formula SrS2O3 xH2O and contains 2.30g of SrS2O3. Find the formula of the hydrate.
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Example 3: The mass of water in a hydrate of MnCl2 xH2O is 36.4g.The total mass of the hydrate is 100g. What is the empirical formula of the hydrate?
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Chapter 3 notes ­ solutions.notebook
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Example 4: A hydrate of zinc nitrate has the formula Zn(NO3)2 xH2O. If the mass of 1 mol of the anhydrous zinc nitrate is 63.67% of the mass of 1 mol of the hydrate, what is the value of x?
Oct 21­8:13 AM
Example 5: A 9.00g sample of calcium phosphate tetrahydrate was thoroughly heated to remove all the water of hydration. What is the mass of the anhydrous calcium phosphate?
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
Practice Problems
page 108 #21
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3.3 Molecular Formula
Suppose a suspect in a theft investigation is a researcher in a biology laboratory. The suspect works with formaldehyde, CH2O. Police officers find traces of a substance at the crime scene and send samples to the Centre for Forensic Science. The forensic analysts find that the substance contains a compound that has an empirical formula CH2O. Will this evidence help to convict the suspect?
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Chapter 3 notes ­ solutions.notebook
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Name Molecular Formula Empirical Formula formaldehyde CH2O acetic acid
C2H4O2
lactic acid C3H6O3
erythrose
C4H8O4
ribose
C5H10O5
glucose
C6H12O6
CH2O Oct 29­8:46 AM
Determining a Molecular Formula
Steps:
1. Find EF
2. Find molar mass of EF
3. Molar mass of molecular formula (usually given in question)
Molar mass of EF
4. Multiply EF by answer to #3
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Chapter 3 notes ­ solutions.notebook
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Example 1: The empirical formula of ribose is CH2O, with a molar mass of 150g/mol. What is the molecular formula?
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Example 2: Determine the molecular formula of capsaicin if it contains 71% carbon, 8% hydrogen, 15.8% oxygen and 14% nitrogen with a molar mass 304g/mol.
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Chapter 3 notes ­ solutions.notebook
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Example 3: The empirical formula of a certain dioxin is C6H2OCl2. If the molar mass of the dioxin is 322g/mol, what is the molecular formula?
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Practice Problems
page 97 #17,18,19
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
The Carbon­Hydrogen Combustion Analyzer
CxHy + O2 CO2 + H2O
A large number of important chemicals are composed of H, C, and O. The carbon­
combustion analyzer is a useful instrument for analyzing these chemicals. (see diagram on page 99)
Oct 29­10:47 AM
After the combustion, all the carbon in the sample is contained in the carbon dioxide and all the hydrogen in the sample is contained in the water. Oct 29­10:53 AM
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Chapter 3 notes ­ solutions.notebook
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Example 1: A 1.000g sample of a pure compound containing only carbon and hydrogen was combusted in a carbon­hydrogen combustion analyzer. The combustion produced 0.6919g of water and 3.338g of carbon dioxide. Determine the empirical formula.
Oct 29­10:56 AM
page 101 #22
Practice page 101 #21
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Chapter 3 notes ­ solutions.notebook
April 01, 2015
• Work on EF of a hydrate lab
• If completed, start chapter 3 worksheet
Oct 30­9:41 AM
Review
• Worksheet on EF and MF
• page 107­108
#6
#11
#12
#14
• Combustion of a 1.086g sample containing carbon and hydrogen is burned in a carbon­
hydrogen combustion analyzer. 2.76g of CO2 and 2.97g of H2O is produced. What is the empirical formula of the compound? C4H21
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