The Mole
Chemistry 2nd semester
Name Period
Teacher
Learning goals for Moles (February)
I will be able to
1. Define andthe following terms: average atomic mass, formula mass, percent composition, mole, Avogadro's number, relative atomic mass, molar mass, empirical formula, and molecular formula Demonstrate that one mole of an element is equal to its relative atomic mass in grams.
2. Calculate the average atomic mass of an element
3. Calculate the formula mass for a compound
4. Demonstrate that one mole of a compound is equal to its molar mass in grams.
5. Explain why Avogadro's number has the value 6.02 x 10 23
6. Demonstrate that one mole of any substance (element, diatomic element, compound)is equal to Avogadro's number of particles.
7. Interconvert between mass, moles, and numbers of particles.
8. Calculate the molar mass of a compound.
9. Calculate the empirical formula of a compound from the percent composition.
10. Calculate the empirical formula of a compound from the mass of each element in the compound.
11. Calculate themolecular formula of a compound from the empirical forumla and the molar mass.
12. Calculate themolecular formula of a compound from the percent composition and the molar mass.
13. Calculate themolecular formula of a compound from the mass of each element in the compound and the molar mass.
14. Calculate thepercent composition of a compound from the molecular formula.
15. Calculate the percent composition of a compound from the masses of elements in the compound.
Reviewing Elements, Compounds, and Counting atoms
Write the letter E in the space if the substance is an ELEMENT.
Write the letter C in the space if the substance is a COMPOUND.
1. H2O 2. NaCl 3. Cl2 4. Fr
5. Al2O3 6. Ir 7. Br2 8. Ca(HCO3)2
9. (NH4)2O 10. N2 11. Mg(OH)2 12. As
Determine and write the number of atoms of each element in the following compounds.
13. NaHCO3 Number of Na atoms Number of C atoms
Number of H atoms Number of O atoms
14. (NH4)2O Number of H atoms Number of N atoms
Number of O atoms
15. Ca(HCO3)2 Number of Ca atoms Number of C atoms
Number of H atoms Number of O atoms
Average Atomic Mass
By the end of this activity you should be able to define and calculate average atomic mass for any element given the mass of its isotopes and percent composition.
You might remember from last semester that we defined the average atomic mass, or you might not. Here is a quick review of atomic structure. Taking the information of a given element from the periodic table we find:
The mass of an atom, as you should remember, is given by the sum of the protons and neutrons.
Isotopes are atoms of the same atom that have different number of neutrons. Another way of saying the same thing would be atoms with the same atomic number but different atomic mass.
Today you will practice differentiating between atomic mass and average atomic mass of objects that will simulate being atoms.
Materials:
· “atoms”: paper clips and pipets of different sizes
· Balance
You will be finding the mass of different objects. Include the appropriate unit for every measurement you make
Procedure
1. Mass independently three small paper clips and three large paper clips and fill in the table
Mass in grams Clip #1 / Mass in grams Clip #2 / Mass in grams Clip #3Small paper clip
Large paper clip
a. What would you say is the mass of small clip, and how consistent is that mass among your small clips?
b. What would you say is the mass of small clip, and how consistent is that mass among your large clips?
2. Find the mass of 9 small paper clips and 1 large paper clip by placing all 10 clips on the balance: ______
a. Calculate the average of one clip among the 10 you massed ______
3. Find the mass of 1 small paper clip and 9 large paper clip by placing all 10 clips on the balance: ______
a. Calculate the average of one clip among the 10 you massed ______
4. If the paper clips were atoms of the same element, how many isotopes would be represented by the 10 paper clips in the two previous cases? ______
Let us try it again with pipettes this time.
5. Mass independently three small pipets and three large pipets and fill in the table
Mass in grams pipette #1 / Mass in grams pipette #2 / Mass in grams pipette #3Small pipette
Large pipette
a. What would you say is the mass of small clip, and how consistent is that mass among your small pipettes?
b. What would you say is the mass of large clip, and how consistent is that mass among your large pipettes?
6. Find the mass of 7 small pipettes and 3 large pipettes by placing all 10 pipettes on the balance: ______
a. Calculate the average of one pipettes among the 10 you massed ______
We can calculate the percentage (%) of a given size of pipettes in the sample we use the following formula:
%= # pipettes of a given sizetoal number of pipettesx 100.
The calculation of the percentage of small pipettes becomes:
%= # small pipettes toal number of pipettesx 100= 7 small pipettes10 total pipettes x 100=70%
b. Calculate the percentage of large pipettes: ______
We can calculate the average atomic mass of the pipettes using the atomic mass for each size of the pipette and their percentage. Imagine a set of 8 small pipettes with a mass of 0.9 g and 2 large pipettes with a mass of 1.6 g. What would the average percentage be?
I. Calculate percentage for each type of pipette:
% small pipettes= 8 small pipettes10 total pipettes x 100=80%
% large pipettes= 2 small pipettes10 total pipettes x 100=20%
II. Calculate the average atomic mass by multiplying the percentage composition for each pipette times its atomic mass, adding the products of the different pipettes and dividing by 100.
Average atomic average atomic mass=% small *atomic mass small+% large *atomic mass large100
=80*0.9 g+20*1.6g100=72g+32100=104g100
Average atomic mass=1.04g
c. Calculate the average atomic mass using percentages and atomic mass: ______
d. Compare the answers you gave to questions 6a. and 6c. Are the values similar or different? Explain your answer in complete sentences
7. Find the mass of 6 small pipettes and 4 large pipettes by placing all 10 pipettes on the balance: ______
a. Calculate the average of one pipette among the 10 you massed ______
b. Calculate the percentage composition for each pipette:
c. Calculate the average atomic mass using percentage and atomic mass
d. Compare the answers you gave to questions 7a. and 7c. Are the values similar or different?
8. If the pipettes were atoms of the same element, how many isotopes would be represented by the 10 pipettes in the two previous cases? ______
9. Describe the difference between the atomic mass of an atom and the average atomic mass of an element:
Propose a method by which you could find the mass of a compound made put of three paper clips and two pipettes. Here is the formula:
2 3
To calculate the mass of a compound made of two pipettes and three paperclips I would …
As a refresher, calculate the oxidation number of the pipette and paper clip given the formula above:
Oxidation number of the pipette: Valence electrons of the pipette:
Oxidation number of the paper clip: Valence electrons of the paper clip
Can you define average atomic mass? ______
Can you calculate the percent composition of an isotope given the number of atoms for each isotope? ______
Can you calculate the average atomic mass for any element given the atomic mass of its isotopes and percent composition?
Let your teacher know if you answer no to any of these three questions
Name
Average Atomic Mass
Reference Reading: Modern Chemistry pg. 77 and 78.
Average Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
1. 100 fifth grades students were weighed at Winnequah and their weights recorded. 77 of the students each weighed 85 lb, while the remaining 23 students each weighed 87 lbs. What is the average weight of these 100 fifth graders? Show your work.
2. If the atomic mass of an atom is simply the sum of the number of protons and neutrons, explain why the atomic mass of any particular element on the periodic table is a decimal.
3. Without doing any math, are there more Br-79 atoms or more Br-80 atoms on earth? (Hint: look at the periodic table.) Explain your answer.
4. Oxygen has 3 isotopes, O-16, O-17, O-18. Again, without doing any math which isotope is the most common? Explain your answer.
For questions 5-12 you must do 5 pts of work. Each Question shows the point value.
5. (1 pt) What is average atomic mass of Lithium if 6.00% exists as Li-6 and 94.00% exists as Li-7?
6. (1 pt) A random sample of the element Boron is found to be 20.0% B-10, and 80.0% B-11. What is boron’s average atomic mass?
7. (1 pt) In a sample of 200 Chlorine atoms, it is found that 110 are Cl-35 and 90 are Cl-36. What is the average atomic mass of Chlorine?
8. (1 pt) Rubidium has two common isotopes, Rb-85 and Rb-87. If the abundance of Rb-85 is 77% and the abundance of Rb-87 is 23%, what is the average atomic mass of rubidium?
9. (1 pt) Neon has two major isotopes, Neon-20 and Neon-22. Out of every 250 neon atoms, 225 will be Neon-20, and 25 will be Neon-22. What is the average atomic mass of Neon?
10. (2 pts) Magnesium has three naturally occurring isotopes. 78.70% of Magnesium atoms exist as Mg-24, 10.03% exist as Mg-25,and 11.17% exist as Mg-26. What is the average atomic mass of Magnesium?
11. (2 pts) Out of 500 silicon atoms, 460 are Si-28, 25 are Si-29 and 15 are Si-30. What is the average atomic mass?
12. (3 pts) What is the average atomic mass of hafnium if, out of every 100 atoms, 5 have a mass of 176 amu, 19 have a mass of 177 amu, 27 have a mass of 178 amu, 14 have a mass of 179 amu, and 35 have a mass of 180 amu? Make sure to show your work.
Formula Mass
By the end of today’s lesson you should be able to define and calculate the formula mass for any compound
Let us start with the idea of Formula Mass with non-chemical examples…
1. Two classmates were complaining about how heavy their backpacks were each day when they went home to study. They decided to weigh their items on a scale and determine who had the heavier backpack.
Naomi is carrying 2 textbooks, 4 notebooks, and a pair of basketball shoes.
Item / Weight (lbs)Textbook / 5
Notebook / 0.4
Basketball shoe (1) / 1
Trumpet / 3.5
Chris is carrying 2 textbooks, 2 notebooks, and a trumpet.
Determine the weight of each bag.
a. Determine the total weight of each item in Naomi’s backpack. Multiply the quantity by each item’s weight from the table.
Item Quantity Weight of Item Total Weight for Each Item
Textbooks X =
Notebooks X =
Basketball Shoes X =
Now, add up the total weight for each item to get the weight of Namoi’s backpack.
b. Use the same steps to determine the weight of the items in Chris’ backpack.
Item Quantity Weight of Item Total Weight for Each Item
Textbooks X =
Notebooks X =
Shoes X =
Now, add up the total weights for each item to get the weight of Chris’ backpack.
c. Which backpack weighs more?
2. The fast food restaurants in the area (McDonalds, Burger King, and Wendy’s) are holding a “Family Pack Sale”. Each food chain had the following Family Packs at a discounted price. Determine the amount of calories in each Family Pack.
The menu shows the calorie amount for ONE item. Use the steps considered in example 1 to fill out the chart.
(Note: It is recommended that the average person consumes about 2,000 calories daily).
McDonalds Menu / CalculationItem / Calories / Calories per Menu Item in Family PK
Big Mac / 540
Chicken Nuggets (10pc) / 470
Large Fries / 500
Large Soda / 310
Total Calories per McDonalds Family Pack =______
A.
McDonalds’ Family Pack includes:
1 Big Mac
2 Chicken Nugget orders (10pc each)
4 Large Fries
4 Large Sodas
Wendy’s Menu / CalculationItem / Calories / Calories per Item in Family PK
The Triple “W” / 1060
Crispy Chicken Sandwich / 700
Large Fries / 500
Large Soda / 310
Total Calories per McDonalds Family Pack =______
B. Wendy’s Family Pack Includes:
2 Triple W’s
1 Chicken Sandwich
2 Large Fries
3 Large Sodas
Burger King Menu / CalculationItem / Calories / Calories per Item in Family PK
Triple Whopper/Cheese / 1230
BK Quad Stacker / 830
Large Onion Rings / 500
Large Soda / 310
Total Calories per McDonalds Family Pack =______
C. Burger King’s Family Pack Includes:
2 Triple Whoppers w/ cheese
2 BK Quad Stackers
3 Large Onion Rings
4 Large Sodas
In the previous activity, we asked you to propose a method to find the mass of a compound using pipettes and paper clips for the following formula:
2 3
In the above examples, we illustrated methods with burgers, fries, and backpacks. What does this have to do with chemistry?