Mole, Schmole…Why The Mole is Important

Moles, Molar Mass, & Conversions

(Chapter 3, pp. 83-87)

1 mole = 6.02 x 1023 particles (atoms, molecules) = molar mass from periodic table (g/mol)

Dozens of Questions

1.  The word DOZEN means that you have 12 of something.

a.  2 dozen eggs means that you have 24eggs.

b.  ½ dozen eggs means that you have 6eggs.

2.  You could have 1 dozen basketballs, and that would mean that you have 12basketballs.

3.  Let’s say you have 1 dozen eggs and 1 dozen basketballs. You have the same number, 12 of each. However, would you expect the 1 dozen eggs to WEIGH the same as the 1 dozen basketballs? no

4.  So, just because you have the same AMOUNT (in this case, 12) of two things, that does/does not (circle one) mean that those two things have the same WEIGHT or MASS.

The Mole

5.  Chemists have come up with their own special number, called the mole (hint: look at the top of this paper). But instead of standing for 12, this number stands for 6.02x1023 (again, look at top of paper).

6.  Is this number BIG/small (circle one)?

7.  The reason that this number is so BIG/small (circle one) is because atoms are BIG/small (circle one). You can’t see just 12 atoms; they are too tiny. But if you have a little/a lot (circle one) of atoms, you CAN see them.

8.  Consider this: If every person living on Earth (assume 6 billion people) worked to count the atoms in one mole of an element, and if each person counted continuously at a rate of one atom per second, it would take all of us 3 million days/months/years (guess and circle one) for all the atoms to be counted!!!!

9.  Obviously, we need a better way than counting to determine the number of atoms in a sample! We can determine the number of atoms by a simple measurement of mass (hint: rhymes with grass).

10.  An Italian scientist by the name of Amadeo Avagadro determined the relationship between the number of atoms and mass. His number, 6.0221415 x 1023 is typically rounded to 6.02 x 1023.

11.  Why this special number? The quantity one mole is set by defining one mole of carbon -12atoms to have a mass of exactly 12 grams (hint: look at periodic table and see which element has an atomic mass close to 12 amu).

12.  So, what does all of this mean? It means that we now have a simple way to count the number of atoms; we only need to know the mass (hint: rhymes with grass) of our sample (and have a periodic table nearby).

It’s a Weight Thing…(Molar Mass)

13.  You would/would not (circle one) expect for 1 dozen eggs to weigh the same thing as 1 dozen basketballs because basketballs are bigger than eggs. Likewise, you would/would not (circle one) expect 1 mole of carbon atoms to weigh the same as 1 mole of hydrogen atoms, because carbon atoms are bigger than hydrogen atoms.

14.  Locate the atomic mass for carbon and hydrogen on your periodic tables. Carbon’s atomic mass is 12.01, and hydrogen’s atomic mass is 1.01. Which weighs more, carbon/hydrogen?

15.  Conveniently, the atomic mass from the periodic table is equal to the molar mass in g/mole (g/mol).

16.  So let’s say you have 1 mole of carbon. That means you have 6.02 x 1023 atoms of carbon, and that, if you weighed it out, you would have 12.01g per mole (g/mol), the molar mass, of carbon.

17.  Let’s say you have 1 mole of hydrogen. That means you have 6.02x1023atoms of hydrogen, and that, if you weighed them out, you have 1.01g/mol, of hydrogen.

18.  The molar mass of boron is 10.81g/mol.

Molar Masses for Compounds

19.  Molar masses for elements are right on the periodic table. However, what about the molar mass for a compound, like NaCl? It’s easy! Na has a molar mass of 22.99g/mol, and Cl has a molar mass of 35.45g/mol. Add them together, and you get 58.44g/mol.

20.  What about MgCl2? In this case, you find that the molar mass of Mg is 24.31g/mol, and that the molar mass of chlorine is 35.45 g/mol. BUT, you have 2 chlorines, so you need to MULTIPLY the molar mass of chlorine by 2 before you add it to the mass of Mg. The molar mass of MgCl2 is 95.21 g/mol.

1 mole = 6.02 x 1023 particles (atoms, molecules) = molar mass (g/mol)

Molar Mass Practice Problems

Determine the molar mass of the following using your periodic table. Remember, the units are g/mol.

1. Li 2. Ca 3. Al 4. Cl2 5. O2

6.94g/mol 40.08g/mol 26.98g/mol 70.90g/mol 32.00g/mol

6. KCl 7. MgI2 8. CCl4 9. Al2O3 10. SiO2

74.55g/mol 278.11g/mol 153.81g/mol 101.96g/mol 60.09g/mol

Conversion Type #1-Moles to Particles and Particles to Moles

YOU MUST USE DIMENSIONAL ANALYSIS TO SOLVE #11-22. ROUND APPROPRIATELY. Use a separate sheet if needed.

Example: How many molecules of NaCl are in 3.00 moles of NaCl?

3.00 moles NaCl x 6.02 x 1023 molecules NaCl = 1.81 x 1024 molecules NaCl

1 mole NaCl

11. How many molecules of NH3 are in 5.0 moles of NH3?

5.0 mole NH3 x 6.02 x 1023 molecules NH3 = 3.0 x 1024 molecules NH3

1 mole NH3

12. How many atoms of barium are there in 2.50 moles of barium?

2.50moles Ba x 6.02x1023atoms Ba =1.51 x 1024 atoms Ba

I mol Ba

13. How many moles is 4.54 x 1022 molecules of H2O?

4.54 x 1022 molecules H2O x ______I mole H2O_____ = 0.0754 moles H2O

6.02 x 1023 molecules H2O

14. How many moles is 1.20 x 1024 atoms of carbon?

1.20 x 1024 atoms C x ____1 mole C = 1.99 mole C

6.02 x 1023 atoms C

Conversion Type #2-Moles to Grams and Grams to Moles

Example: How many grams of NaCl are in 8.00 moles of NaCl?

8.00 moles NaCl x 58.44 g NaCl = 468 g NaCl

1 mole NaCl

15. How many grams of Al2O3 are in 5.55 moles of Al2O3?

5.55 moles Al2O3 x __101.96 g = 566 gAl2O3

1 mole Al2O3

16. How many grams of boron are in 6.75 moles of boron?

6.75 mole B x 10.81g B = 73.0 g B

1 mole B

17. How many moles of copper is 127.1 g of copper?

127.1g Cu x 1 mole Cu = 2.000 mole Cu

63.55g Cu

18. How many moles of KI is 497.7 g of KI?

497.7g KI x 1 mole KI = 2.998 mole KI

166g KI

Conversion Type #3-Grams to Particles and Particles to Grams

Example: How many molecules of NaCl make up 232 g of NaCl?

232 g NaCl x 6.02 x 1023 molecules of NaCl = 2.39 x 1024 molecules of NaCl

58.44 g NaCl

19. How many molecules of H2O make up 36.0 g of H2O?

36.0g H2O x 6.02 x 1023 molecules = 1.20 x 1024 molecules H2O

18.02g H2O

20. How many atoms of phosphorous are there in 90.0 g of phosphorous?

90.0g P x 6.02 x 1023 atoms P = 1.75 x 1024 atoms P

30.97g P

21. How many grams of Fe2O3 would you need to have 3.01 x 1023 molecules of it?

3.01 x 1023 molecules Fe2O3 x _____159.70 Fe2O3 = 79.9g Fe2O3

6.02 x 1023 molecules Fe2O3

22. How many grams of silicon would you need if you wanted 3.01 x 1024 atoms of it?

3.01 x 1024 atoms Si x ______28.09g Si = 140. g Si

6.02 x 1023 atoms Si