The Mole

Review of Terms Atomic mass: the weighted average of the masses of the existing isotopes of an element Formula mass: the sum of all of the atomic masses of all atoms in a compound as represented in a chemical formula Atomic mass units (AMUs): 1/12 the mass of a carbon-12 atom; unit of measurement for the mass of an atom

Counting There are many things that are too numerous to count. Can you think of any?

Scientists’ Problems Problem 1: There are too many atoms in a sample of a substance to count them all. Problem 2: Atomic and formula masses are measured in AMUs, but scientists measure mass in grams in the laboratory.

What They Needed A: A way to count atoms without having to count them individually. B: A value to relate AMUs to grams.

Question: How many atoms are necessary to make up a mass in grams that is equal to an element’s atomic mass in AMUs?

Solution:

What’s a Mole? A mole (mol) is a number equal to the number of carbon atoms in exactly 12 grams of pure carbon-12 = Avogadro’s number. It is a unit, like a dozen or a ream, used to represent a particular number.

What’s a Mole? So just how many atoms are in 12 grams of carbon-12? x How big is that? Well, a mole of marbles would cover the earth in a stack 50 miles deep!

Representative Particles 1 mole always equals 6.02 x of something…but what? A mole of a(n) …= 6.02 x elementatoms ionic compoundformula units molecular substancemolecules

Molar Mass So, the atomic mass of an element is also the mass of 1 mole (6.02 x atoms) of that element 1 mole of H = grams of H = 6.02 x H atoms

Molar Mass The mole establishes a relationship between AMUs and grams. Molar mass: The mass in grams of one mole of a compound. NaCl Formula mass = AMU Molar mass = g/mol

Calculating Molar Mass CH 4 Mass of 1 mol of C = 1 x g = g Mass of 4 mol of H = 4 x g = 4.03 g Mass of 1 mol of CH 4 = g Molar Mass of CH 4 = g/mol

What We Know About the Mole It establishes a relationship between atomic mass units and grams. 1 mole = 6.02 x representative particles

Analogies Equalities can act as conversion factors between units. How many eggs are there in 5½ dozen eggs? 5.5 dozen x = 66 eggs How many nickels are in 11 rolls that each contain 40 nickels? 11 rolls x = 440 nickels 12 eggs 1 dozen 40 nickels 1 roll

Conversion Factors How can the mole be used for conversions? Moles and mass Moles and # of rep. particles

Moles  Mass Conversion factor: Molar Mass The mass of one mole of a substance can be used to determine the mass of a sample if the number of moles is given. The mass of one mole of a substance can be used to determine the number of moles making up a sample if the mass in grams is given.

Moles  Mass Calcium carbonate, CaCO 3 (also called calcite), is the principal mineral found in limestone, marble, chalk, pearls, and the shells of animals such as clams. a.Calculate the molar mass of calcium carbonate. b.A certain sample of calcium carbonate contains 4.86 mol. What is the mass in grams of this sample?

Moles  Mass Juglone, a dye known for centuries, is produced from the husks of black walnuts. It is also a natural herbicide (weed killer) that kills off competitive plants around the black walnut tree but does not affect grass and other noncompetitive plants. The formula for juglone is C 10 H 6 O 3. A sample of 1.56 g of pure juglone was extracted from black walnut husks. How many moles of juglone does this represent?

Moles  Representative Particles Conversion Factor: Avogadro’s Number Avogadro’s number can be used to calculate the number of particles in a given number of moles of a substance. Avogadro’s number can be used to calculate the number of moles made up by a given number of particles of a substance.

Moles  Representative Particles Sucrose, or table sugar, has the formula C 12 H 22 O 11. If 0.85 mol of sucrose are used in a recipe, how many molecules of sucrose are being added?

Moles  Representative Particles Formaldehyde, H 2 CO, was used in the past to preserve biological samples. How many moles of formaldehyde does 1.51 x molecules represent?