Download Unit 8- The Mole

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Unit 8- The Mole
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Description
Warm-up
Notes on the Mole
Mole Map
The Mole
Chemical Quantities
Percent Composition
Empirical and Molecular Formula
Notes
Empirical and Molecular Formulas
Practice
Hydrates Notes
Hydrates
Water in a Hydrate
Unit 8 Test Review
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Unit 8 Test
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Water in a Hydrate Formal Lab
Report
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Date
Completed
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In class on 2/7 and
2/8
Due on Moodle 2/9
(A day) and 2/10 (B
day) at 11:59 pm
Warm-up
Day 1
1. If you are making 113 dozen mini cupcakes, and each batch makes 60 mini cupcakes,
how many batches will you need to make?
Day 2
1. How many atoms are in 2.5 moles of silver?
2. What is the molar mass of calcium phosphate?
Day 3
1. What is the empirical formula for a compound which contains 15.77 % aluminum, 28.11
% sulfur, and 56.12 % oxygen?
Day 4
1. Magnesium chloride froms a hydrated salt. When a sample of magnsium chlorideis
analyzed, it is found to contain 53.20 % water.
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Notes on the Mole
• One mole is defined as the number of atoms in 12 g of ________________.
• A mole is a counting number. One mole is a specific number of atoms,
molecules or formula units. Avogadro’s number is ___________________.
• Now we can relate the atomic mass unit to the gram. 1 atom of carbon 12
has a mass of 12.01 amu. 6.02*1023 atoms of carbon-12 have a mass of
12.01 g. 1 mole of carbon atoms have a mass of 12.01 g.
• The mass on the periodic table is known as the Molar Mass. It has units of
g/mol.
Calculating the Molar Mass of a Compound
1. Write the formula
2. Calculate the mass by adding up the masses of each element in the
compound.
3. Make the unit g/mol.
• Example: Calculate the mass of one mole of calcium chloride.
o Write the formula
o Calculate the mass
o Change the unit
• Ex: Barium nitrate
• Ex: Xenon tetrachloride
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• You can use the molar mass and Avogadro’s number to perform
conversions.
• Calculate the number of formula units present in 3.45 moles of barium
nitrate.
• Calculate the number of molecules in 19.3 moles of ammonia (NH3).
• Calculate the mass of 7.89*10-20 atoms of oxygen.
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The Mole Practice
Perform the following conversions, using dimensional analysis.
1. Calculate the mass of 1.73 mol of zirconium (Zr).
2. What is the mass in grams of 3.50 moles or the element copper?
3. Calculate the number of moles of zinc (Zn) in a sample that weighs 143
grams.
4. A chemist produced 11.9 grams of AL. How many moles of aluminum were
produced?
5. Calculate the number of atoms of in a sample of calcium (Ca) with a mass of
6.14 grams.
6. Determine the number of atoms in 1.50 grams of carbon.
7. Calculate the mass of a sample of platinum (Pt) that contains 2.15 x 1024
atoms.
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Chemical Quantities
Show ALL work for the following problems!!
1. 45.3 g of Nitrogen is how many moles?
2. How many molecules are in 112.5g of dinitrogen tetroxide.
3. What is the mass of 6.70 x 1024 formula units of Lead (IV) Oxide?
4. How many molecules are in 56.2g of carbon dioxide?
5. How many molecules are in 36.5 g of water?
6. What is the mass of 4.67 x 1023 molecules of xenon pentafluoride?
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PERCENT COMPOSITION
Percent composition refers to what percent of the compound is made up of each
element. To calculate the percent composition, divide the mass of the element
by the molar mass of the compound. If there is more than one atom of an
element, you must multiply the molar mass by the number of moles. Let’s
calculate the percent composition of Oxygen and Hydrogen in water.
Molar mass of oxygen: 16.00 g
Molar mass of hydrogen: 1.008 g
Molar mass of water: 18.016 g
The percent composition of oxygen is:
.
.
= 0.8881
88.81 %
There are two hydrogens in every molecule of water, so you must multiply the
molar mass of hydrogen by two.
2 * 1.008 g = 2.016 g
The percent composition of hydrogen is:
.
.
= 0.1119
11.19 %
The two percents add up to 100 %, a good indication that you did the problem
correctly ☺
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Empirical and Molecular Formula Notes
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•
•
Empirical Formulas- Indicate the ratio of atoms present in a compound.
Molecular Formulas- Indicate the exact number of atoms present in a compound.
Ex: Glucose
Steps to calculate an empirical formula:
1. Start with the mass of each element given in the problem. (If percents are given,
assume the sample has 100 g, and change percentage unit to unit of grams.)
2. Convert to moles.
3. Divide each by the smallest number of moles to get the ratio. (If necessary, multiply
to get whole numbers.)
4. Answers in #3 become subscripts.
•
Ascorbic Acid (vitamin C) is 40.92% Carbon, 4.58% Hydrogen and 54.50% Oxygen by
mass. Calculate the empirical formula for Ascorbic Acid.
•
A 5.325 g sample of methyl benzoate, a compound used in the manufacture of
perfumes, is found to contain 3.758 g of carbon, 0.316 g of hydrogen and 1.251 g of
oxygen. What is the empirical formula of methyl benzoate.
Steps to calculate a molecular formula:
1. Calculate the empirical formula.
2. Calculate the molar mass of the empirical formula.
3. Divide the mass of the molecular formula (given) by the mass of the empirical
formula.
4. Multiply the subscripts in the empirical formula by the answer in step 3.
•
Mesitylene, a hydrocarbon that occurs in crude oil, is 89.92% carbon. The molar mass of
this compound is 121 g/mol. Calculate the empirical and molecular formulas of this
substance.
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Empirical and Molecular Formulas Practice
1. What is the empirical formula of a compound that is 25.9% nitrogen and 74.1%
oxygen?
2. What is the empirical formula of a compound that is 79.9% carbon and 20.1%
hydrogen?
3. What is the empirical formula of a compound that is 17.6% Na, 39.7% Cr and
42.7% O?
4. You find that 7.36g of a compound contains 6.93g of oxygen while the rest is
hydrogen. If you know the molar mass of the compound is 34.0g/mol what is the
molecular formula?
5. What is the molecular formula of a compound that is 54.5% carbon, 13.6%
hydrogen and 31.8% nitrogen? Its molar mass is 88g/mol.
6. What is the molecular formula of a compound that is 40.0%C, 53.4%O and
6.6%H. Its molar mass is 120 g/mol.
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Hydrates Notes
• A hydrate is a salt with a specific number of water molecules around it.
o To name a hydrate, name the salt, then use the covalent pre-fixes to
indicate the number of waters. Follow the pre-fix with the word
“hydrate.”
• EX: MgSO4∙7H2O (Magnesium sulfate heptahydrate)
• Remember the formula for percent error!
• The percent of calcium in calcium chloride was experimentally determined
to be 32.1%. Calculate the % error.
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Hydrates
1. Name the following hydrates.
a. NaC2H3O2 • 3H2O
b. Ba(OH)2 • 8H2O
c. CoCl2 • 6H2O
2. Write the formula for the following hyrdrates.
a. Calcium chloride dihyrdrate
b. Ammonium oxalate monohydrate
c. Iron (III) Phosphate hexahydrate
3. Calculate the % water in each of the following hydrates.
a. Sodium carbonate decahydrate
b. Barium hydroxide octahydrate.
c. Strontium chloride hexahdrate.
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4. A hydrated sodium tetraborate is commonly called Borax. Chemical analysis indicates that
this hydrate is 52.8% sodium tetraborate [Na2B4O7] and 47.2% water. Determine the formula
and name the hydrate.
5. A 1.628 g sample of a hydrate of magnesium iodide is heated until its mass is reduced to
1.072 g and all water has been removed. What is the formula of the hydrate?
6. The data table below shows data from an experiment to determine the formulas of hydrated
barium chloride. Determine the formula for the hydrate and the name of the hydrate.
Mass of empty crucible
Mass of hydrate and crucible
Initial mass of hydrate
Mass after heating
Mass of anhydrate
21.30 g
31.35 g
29.87 g
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WATER IN A HYDRATE
INTRODUCTION
Many compounds are formed as a result of reactions that occur in water solutions. These
compounds appear to be dry, but when they are heated, large amounts of water are released. The
water molecules are part of the crystalline structure and are weakly bonded to the ions or
molecules that make up the compound. Such compounds are known as hydrates, meaning that they
contain water. The solid that remains when the water is removed is referred to as the anhydrous
salt, or anhydrate.
Hydrate + heat Anhydrate + water
Usually, the amount of water present in a hydrate is in a whole number molar ratio to the moles of
anhydrate. An example of a hydrate is magnesium sulfate. Its formula is MgSO4 · 7H2O, indicating
that seven moles of water are combined with one mole of magnesium sulfate in the crystalline form.
In this investigation you will be given an unknown hydrate and asked to determine the percent of
water in the compound. From this information, the molar ratio of water to anhydrous salt will be
calculated. Finally, the identity of the hydrate will be determined.
PURPOSE
To determine the percent composition of water in Copper (II) Sulfate Pentahydrate.
PRE – LAB DISCUSSION
Answer all questions in complete sentences. Work for a calculation is required.
1. What is a hydrate? An Anhydrate?
2. Look up the MSDS for Copper (II) Sulfate Petahydrate. What are the safety precautions?
3. What is the molar mass of the hydrate of magnesium sulfate described in the introduction?
4. What is the percent of water in MgSO4 · 7H2O?
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PROCEDURE
1. Heat a clean and dry evaporating dish for three minutes and then allow to cool for an additional
three minutes.
2. Record the mass of the evaporating dish.
3. Add a small scoop of Copper (II) Sulfate Pentahydrate to the evaporating dish and record the
mass.
4. Heat the evaporating dish and hydrate until there is no more popping or spattering. Be careful
not to burn the salt.
5. Allow the evaporating dish to cool and then record the mass.
6. Reheat the crucible and salt for two minutes and then cool and record the mass again.
7. The final two masses should be within 0.02 g of one another. If they are not, repeat the last two
steps.
8. Dispose of the salt in the appropriate waste container in the fume hood and clean your work
area. Be sure to wash your hands with soap and water.
DATA TABLE
Mass of evaporating dish
Mass of evaporating dish and hydrate
Mass of evaporating dish and anhydrate
Mass of evaporating dish and anhydrate
CALCULATIONS
Calculate the initial mass of hydrate
Calculate the mass of water lost
Calculate the moles of water lost
Calculate the mass of the anhydrate
Calculate the moles of the anhydrate
Calculate the % water in your hydrate
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CRITICAL THINKING: ANALYSIS AND CONCLUSIONS
1. Calculate the theoretical % water in your compound AND the percent error for the experimental
% of water in your hydrate.
2. Explain what effect the following errors would have on the value for the percent water in the
hydrate. (i.e. would the following errors resulted in too high a % water or too low % water)
a. The hydrate was not heated long enough to drive off all the water.
b. A damp crucible was used and was not dried before adding the hydrate.
c. The crucible and contents were allowed to cool overnight before finding the mass.
Formal Lab Report
Submitted
Formatting
Numbers
Information
Introduction
Procedure
Data Tables
Calculations
Results and
Conclusion
Description
All papers must be submitted on Moodle
Title, Date, Lab Partners
All numbers taken to the appropriate
number of sig figs AND have units
Purpose, Hypothesis, Materials
Answers to the pre-lab questions (in
paragraph form)
Written in the past tense, 3rd person. In
paragraph form.
The two tables from the lab must be retyped and added to the lab
Show your work for the calculations you
performed in this lab (for the calculations
table and number one of critical thinking)
Discuss possible sources of errors in the lab,
if your results were what you expected- be
sure to answer critical thinking question
number 2
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Points
5
6
8
6
15
10
20
20
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Unit 8 Test Review
1. The molecular formula of allicin, the compound responsible for the
characteristic smell of garlic, is C6H10OS2.
(a) What is the molar mass of allicin?
(b) How many moles of allicin are present in 5.00 mg of this substance?
(c) How many molecules of allicin are present in 5.00 mg of this substance?
(d) How many sulfur atoms are present in 5.00 mg of allicin?
2. A sample of glucose, C6H12O6, contains 5.77 x 1020 atoms of carbon.
(a) How many atoms of hydrogen does the sample contain?
(b) How many molecules of glucose does the sample contain?
(c) What is the mass of the sample?
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3. Calculate the percent composition of Oxygen in the following compounds:
calcium carbonate, lead (II) hydroxide, acetic acid, and magnesium phosphate.
4. Adrenaline, a hormone secreted into the bloodstream in times of danger or
stress is 59.0% carbon, 7.1% Hydrogen, 26.2% oxygen and 7.7% nitrogen. The
molar mass of adrenaline is 180 g/mol. Calculate the molecular formula of
adrenaline.
5. The characteristic odor of pineapple is due to ethyl butyrate, a compound
containing carbon, hydrogen and oxygen. Combustion of 2.78 mg of ethyl
butyrate produces 6.32 mg of CO2 and 2.58 mg of water. The molar mass of ethyl
butyrate is 116.16 g/mol. Determine the molecular formula of ethyl butyrate.
6. Epsom salts, a substance used as a strong laxative by veterinarians, is a hydrate
of magnesium sulfate. When 5.061 g of the hydrate is heated all the water of
hydration is lost leaving 2.472 g of the anhydrate. Determine the formula of this
hydrate and name the hydrate.
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