1 mole = (Avogadro s Number)

Size: px

Start display at page:

Download "1 mole = (Avogadro s Number)"

Elisabeth Gibbs
5 years ago
Views:

1 SCHM 109-MWF Mole Pre-Activity Name This Pre-Activity is due Wednesday, September 4, 008! I. Introduction One of the hardest concepts for chemistry students to grasp is the mole, and the calculations that result from it. An understanding of the mole is extremely important for success in chemistry because not only is it the SI unit for quantity, but it also is the basis for most stochiometric calculations. When a chemical equation is balanced, the numerical coefficients in front of each reactant or product represent the number of moles. The determination of limiting reagents, percent yields, dilution percentages, and even molecular formulas themselves are all based on the mole concept. The study of the mole relates to many mathematical functions. Throughout a unit on the mole, you would use dimensional analysis, which is the converting of one unit to another unit through the use of conversion factors. You will learn to relate the mole to gram formula mass, and number of particles. Many students find the mole concept difficult to grasp. One reason for this is because the mole is such an extremely large number. After all, it is difficult to imagine 60,00,000,000,000,000,000,000 particles, at least not individually! II. Mole Introduction The SI unit mole refers to the number of particles (atoms, ions, or molecules) equal to the number of atoms in exactly 1 grams of the 1 C isotope of carbon. By definition there are particles in 1 mole. This humongous number is called Avogardro s number. A more elementary way of stating this is 1 mole = (Avogadro s Number) Sounds strange? Well let s work with something familiar. There are many kinds of measurements in the world. Since the mole indicates the quantity of particles, let s start with a unit that deals with the number of items. One that we are familiar with is the dozen. We all know that a dozen = 1 This can be a dozen of anything. A dozen eggs, a dozen feathers. How many eggs are in 1 dozen eggs? You know how to do this, it is easy. 1 dozen 1 eggs = eggs 1 dozen How many feathers are there in 6 dozen? 6 dozen 1 feathers = feathers 1 dozen

2 Now returning to the mole, let s do a similar calculation How many molecules are in 3 moles? This is easy, just like before. 3 moles 6.0 X 10 3 molecules = molecules 1 mole How many donuts are in 7.5 moles? 7.5 moles 6.0 X 10 3 donuts = donuts 1 mole Let s change it up a bit How many moles are in 8.7 x 10 3 Na + ions? 8.7 x 10 3 Na + ions 1 mole = moles 6.0 x10 3 Na + ions Now you try some on your own using dimensional analysis How many moles are in 3.57 x 10 3 newspapers? How many diamonds are in 75 moles? How many moles are in 3.7 x Fe atoms? How many H O molecules are in 39.6 moles?

III. Relationship between Mole and Mass Now what is neat about the mole is that we can relate it to mass. But be careful! It is important that we never forget the mole indicates quantity!

3 III. Relationship between Mole and Mass Now what is neat about the mole is that we can relate it to mass. But be careful! It is important that we never forget the mole indicates quantity! Let s go back to our familiar unit the dozen. How many feathers are there in 1 dozen? How many TV s are there in 1 dozen? How many physical science textbooks are there in 1 dozen? Does 1 dozen feathers weigh the same as 1 dozen TV s? Does 1 dozen TV s weigh the same as 1 dozen physical science textbooks? Does 1 dozen physical science textbooks weigh the same as 1 dozen feathers? Is there a relationship between 1 dozen and weight? What do 1 dozen feathers, 1 dozen TV s, and 1 dozen physical science textbooks share in common? I realize this is redundant but it is important that we understand that 1 dozen does not give ANY indication about how much something weighs! All it tells is how many objects we have. The mole works the exact same way! Since this is chemistry, let s relate the mole to mass On the periodic table, each block contain three things. This should be review. Label what each represents. If you cannot remember look in your Lecture V notes. How is atomic weight different from the mass number? (This should be review. If you do not remember look in Lecture V notes.)

4 There are two units for atomic weight. Atomic mass unit (abbreviated amu) and grams/mole (abbreviated g/mol). We will focus on the g/mol. So the atomic weight of Zirconium in 91.4 g/mol. How many grams of Zirconium atoms are in 1 mole? g 914. =? mol Multiply both sides by 1mol 91.4 g = 1mol So the atom weights in the periodic table are equivalent to 1 mol! How would we set this up using dimensional analysis? Well first we start with what is given 91.4 g Zr 1 mol Zr = 91.4 g Zr Now you try Using dimensional analysis, how many grams of Fe atoms are in 1 mole? Using dimensional analysis, how many grams of Xe atoms are in 1 mole? Now here is the real test. Even though 1 mole of Fe atoms weights different from 1 mole of Xe atom, would you predict number of atoms contained in this amount to be the same? If your answer is yes, state specifically how many. Now what if you were asked the following: How many moles are in 45.6 g of Zr? We can solve this using dimensional analysis because we know that 1 mol Zr = 91.4 g Zr! 45.6 g Zr = mol Zr = 91.4 g Zr mol Zr

5 What if you were asked how many Zr atoms are contained in 45.6 g of Zr? We have done this earlier so this is easy! We use Avogadro s number! (1 mol = ) 45.6 g Zr 91.4 g Zr Zr atoms = Zr atoms = Zr atoms Using dimensional analysis, try these How many moles of are in 4.5 g Fe? How many Xe atoms are in g of Xe? IV. Molecules So we talked about atoms but what about molecules? How do we determine how grams of H O are in 1 mole? This is easy because we have the recipe in the chemical formula! We know that water is comprised of hydrogen atoms and 1 oxygen atom. I know what the atomic weight of these are because I can look this up on the periodic table. By taking the sum of these weights, I have determined the formula (aka molecular) weight of water! The units for formula weight is g/mol! Atomic weight: oxygen = g/mol and hydrogen = g/mol Therefore the formula weight of water is g/mol + ( g/mol) = g/mol = g/mol Rearranging this formula weight we come to a similar conclusion (refer back to Zr example) that g H O = 1 mol How would we set this up using dimensional analysis? g HO 1 mol H O = g HO 1mol H O

6 Now you try (Remember to determine the formula weight first!) Using dimensional analysis, how many grams of CH O molecules are in 1 mole? Using dimensional analysis, how many grams of Cl molecules are in 1 mole? Now here is the real test. Even though 1 mole of CH O molecules weights differently from 1 mole of Cl molecules, would you predict number of molecules contained in this amount to be the same? If your answer is yes, state specifically how many. What about this? Even though 1 mole of Fe atoms weights differently from 1 mole of CH O molecules, would you predict number of atoms/moelcules contained in this amount to be the same? If your answer is yes, state specifically how many. Now what if you were asked the following: How many moles are in 89.7 g of H O? 1mol HO 89.7 g H O = mol HO = 4.98 mol HO g H O What if you were asked how many H O molecules are contained in 89.7 g of H O? We have done this earlier so this is easy! We use Avogadro s number! (1 mol = ) 89.7 g H O 1mol HO HO molecules g H O 1mol H O 3 = H 4 = H O molecules O molecules

7 Using dimensional analysis, try these (Since these are molecules don t forget to determine the formula weight first!) How many moles of are in 7.0 g CH O? How many Cl molecules are in g of Cl? If you understand this, you are now ready for tomorrow s activity.

Atoms, Molecules, and the Mole

Atoms, Molecules, and the Mole The Mole Now that we know how to write and name chemical compounds, we need to understand how chemists use these formulas quantitatively. As chemists, we need to know how many atoms or molecules are reacting