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The Mole
Video on moles:
1. What is a “mole” used for in chemistry?
2. What is Avogadro’s number? _________________________________
3. What is another name for Avogadro’s number? ___________________
4. Why are moles useful in science? What’s the purpose of a mole?
5. How is a “mole” similar to a “dozen”?
Mole =
MOLE
1. How many candies are in a mole of candies?_____________________________________________________
2. How many molecules are in a mole of water, H2O? _______________________________________________
3. How many atoms are there in a mole of sodium? _________________________________________________
4. Why do scientists use moles? _________________________________________________________________
____________________________________________________________________________________________
5. One mole of hydrogen atoms has a mass of 1.01 grams. How many atoms would be in 3.03 grams of
hydrogen? How do you know? ________________________________________________________________
6. One mole of oxygen atoms has a mass of 15.99 grams. How many atoms would be in 31.98 grams of oxygen?
How do you know? _________________________________________________________________________
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Molar mass
So, now we know that moles come in handy when we are talking about mass. Who wants to talk about the mass
of one individual thing when you can talk about the mass of 602,000,000,000,000,000,000,000 of them?! We
saw that a mole of hydrogen atoms had a mass of 1.01 g. The mass of a mole of something is called the molar
mass (see the word “mole” in “molar”?).
Let’s make up a word for a moment = “dozenar mass”. This term would refer to the mass of a dozen of
something. A dozen golf balls would have a mass of 62 grams, meaning the dozenar mass of golf balls is 62
g/dozen. A dozen cotton balls, however, would have a mass of 10 grams, meaning the dozenar mass of cotton is
10 g/dozen. See, we can use molar mass to talk about anything—hot dogs, H2O compounds, lone gold atoms,
whatever, as we are just talking about the mass of 6.02 x 1023 of those things.
1. What is molar mass (in your own words)? _______________________________________________________
____________________________________________________________________________________________
____________________________________________________________________________________________
2. How is it possible that a mole of hydrogen has a different molar mass than a mole of carbon atoms – even
though both moles have 6.02 x 1023 atoms? _____________________________________________________
_________________________________________________________________________________________
3. Which would have a larger mass: a mole of baseballs or one bowling ball? Why do you think this?
____________________________________________________________________________________________
____________________________________________________________________________________________
I’m ready to move on Ms. Wilson!
How do we calculate the molar mass??
One of the most important skills in chemistry is to be able to calculate the molar mass of an element or
compound. First, let’s take a look at the periodic table (YAYYY!!!! WE GET TO USE PERIODIC TABLES AGAIN!!! I
know it is quiet reading time, but I understand if you cheer with excitement at this point). The molar mass of an
element is located at the bottom of the periodic table. For example, Hydrogen has a molar mass of 1.01
gram/mol (pronounced “grams per mole”). This means that 1 mole of hydrogen atoms (6.02 x 1023 Hydrogen
atoms) has a mass of 1.01 grams. The label for molar mass is “grams per mole” and is written g/mol. This label
helps us remember that the molar mass is the # of grams in each mole of the substance.
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Molar mass
Finding the molar mass of individual elements:
4.
What is the molar mass of magnesium
(include the units)? ________________________
5.
What is the molar mass of chlorine atoms
(include the units)? _________________________
6.
What is the molar mass of lithium atoms
(include the units)? __________________________
Ok, that was pretty easy. But how do we calculate the molar mass of a compound?
We need to know how to use the molar masses of individual elements to determine the molar masses of whole
compounds. We calculate the molar mass of a compound by adding the masses of the atoms that make it up. For
example, the molar mass of H2O would be the molar mass of 2 hydrogen atoms and 1 oxygen atom added
together. Each hydrogen atom has a molar mass of 1.01 g/mol. Oxygen has a molar mass of 16.00 g/mol.
EXAMPLE: Find the molar mass of 1 mole of H2O
STEP 1: Use the periodic table to find the molar mass of each element in the compound (you may round)
Molar mass of Hydrogen = 1.01 g/mol
Molar mass of Oxygen = 16.0 g/mol
STEP 2: Use the coefficients to figure out how many of each atom is in 1 molecule. Then multiply the molar
mass of each element by how many of each element you have
H2O = has 2 hydrogen atoms and 1 oxygen atoms
For the 2 hydrogen atoms…
2 x 1.01 g/mol H = 2.02 g/mol
For the 1 oxygen atom…
1 x 16.00 g/mol O = 16.00 g/mol
STEP 3: Add together the masses from each element to get the molar mass of the compound
Adding these together to make H2O….
2.02 g/mol + 16.00 g/mol = 18.02 g/mol H2O
Molar mass of H2O = 18.02 g/mol
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In picture form:
This, of course, means that 6.02 x 1023 little molecules of water have a mass of 18.02 grams.
7. Describe in words how you calculate the molar mass of a compound?
_________________________________________________________________________________________
_________________________________________________________________________________________
_________________________________________________________________________________________
8. What is the molar mass of CaCl2?
STEP 1: Count the atoms of each element in CaCl2
A) There are _______ Ca atoms
B) There are _______ Cl atoms
STEP 2: Find the molar mass of each element from the periodic table
A) The molar mass of Ca is __________________________ g/mol
B) The molar mass of Cl is __________________________ g/mol
CHECK!! WHY is the label for molar mass “g/mol” ___________________________________________________
Multiply the molar mass of each element by how many there are in the compound below:
(Need help?? Step 1 (A) x Step 2 (A) = …)
STEP 3: Add together the masses from each element to find the molar mass
FINAL ANSWER (include units) = ________________________________________
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Try it on your own now!!
9. What is the molar mass of C2HF3? (Show your work below. Include units.)
10. What is the molar mass of Mg(OH)2? (Show your work below. Include units)
11. What is the molar mass of Na(PO4)2? (Show your work below. Include units)
12. What is the molar mass of (NH4)2O? (Show your work below. Include units)
I got this Ms. Wilson!
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Got it? Let’s practice and see
Solve the following problems. If you have any questions, just raise your hand and ask!
1) Molar mass of CO2
Molar mass carbon = ___________ Molar mass oxygen = ___________
Molar mass CO2 =
2) Molar mass of C2H4O2
Molar mass carbon = _______ Molar mass hydrogen = ________ Molar mass oxygen = _______
Molar mass C2H4O2 =
3) Molar mass of Al2(S2O3)3
Molar mass aluminum = _______ Molar mass sulfur = ________ Molar mass oxygen = _______
Molar mass Al2(S2O3)3=
On a separate sheet of paper, show your work for the following problems! You will turn
in your work and answers for a class grade. Make sure to label your final answer!!
Molar Mass of Compounds
Compound
Molar Mass (g/mol)
Compound
1. KI
7. H2O
2. NaCl
8. Na(OH)
3. CaBr2
9. Mg(SO4)
4. Cl2
10. Ag3(PO4)
5. Al2O3
13. C6H12O6
6. Fe2S3
14. Fe(NO3)3
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Molar Mass (g/mol)
Mixed Practice – moles and molar mass
We will be able to define a mole and Avogadro’s number:
1. How many molecules are in a mole of NaCl? __________________
2. How many sodium atoms are in a mole of Na2O atoms? _____________________ Explain in
words how you got this answer.
3. How many chlorine atoms are in a mole of AlCl3 atoms? _____________________ Explain in
words how you got this answer.
We will be able to define and calculate the molar mass of elements and compounds
4. What is the definition of molar mass? Explain in your own words.
Show your work (on the back/separate sheet as needed) for the questions below. Label your final answer with
units!!
5. What is the mass of one mole of lithium?
__________________
6. What is the mass of one mole of calcium?
__________________
7. What is the mass of one mole of silver?
__________________
8. What is the molar mass of one mole of fluorine, F2? __________________
9. What is the molar mass of one mole of KBr?
__________________
10. If the molar mass of hydrogen in 1.01 grams, how many moles of hydrogen are in 6.06 grams? ____________
11. If the molar mass is of nitrogen is 15 grams, how many moles of hydrogen are in 7.5 grams? ______________
12. What is the mass of 4 moles of lithium?
16. What is the mass of 7 moles of ammonium oxide
(NH4)2O ?
13. What is the mass of 7 moles of strontium?
17. What is the mass of 5 moles of hydrochloric acid
(HCl)?
14. What is the mass of 2 moles of KBr?
18. How many TOTAL atoms are in 3 moles of NaCl?
19. How many TOTAL atoms are there in 2 moles of H2O?
15. What is the mass of 3 moles of NaCl?
20. How many TOTAL atoms are there in 4 moles of
Mg(OH)?
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