Mole Concept p5944

Introduction to Stoichiometry Molecular weight Formula Mass Molecular Mass Avagadro’s Number Moles

Atomic Mass: Where can you find it? • An atomic mass unit is defined as 1/12 the weight of the carbon-12 isotope. The old symbol was amu, while the most correct symbol is u (a lower case letter u). • The atomic mass of any element is the “average” atomic mass and found on the periodic table.

Molecular Weight, Formula Mass and Molar Mass • The molecular weight of a substance is the weight in atomic mass units of all the atoms in a given formula. • The formula mass of a substance is the mass in amu of all the atoms in a given formula • The molar mass is the same as molecular weight or formula mass but is described as the mass of one “mole” of a substance.

How to calculate the molecular weight: Point 1 • You need to know how many atoms of each element are in a substance in order to calculate its molecular weight.

Example: • For example H2O has two atoms of hydrogen and one atom of oxygen. H2O2 has two atoms each of oxygen and hydrogen. Mg(OH)2 has one atom of magnesiun and two each of oxygen and hydrogen.

How can you tell how many atoms are in a formula? • If a subscript follows an atom with no parenthesis, that number tells you how many of that atom are present. If parentheses are involved, you must multiply each subscript inside by the one which is outside.

Examples: • How many of each element are in the following examples: • KCl • Fe2O3 • Al(NO3)3 • NH4NO3 • Al2(SO4)3

Answers: • KCl -- K - one; Cl - one • Fe2O3 -- Fe - two; O - three • Al(NO3)3 -- Al - one; N - three; O - nine • NH4NO3 -- N - two; H - four; O - three • Al2(SO4)3 -- Al - two; S - three; O twelve

After finding the number of atoms, you need to find the atomic mass of each element – #2 to know the atomic •Point You need weight of each element in order to calculate the molecular weight of the substance. • The atomic weight of each element is found by examining the periodic table.

How to calculate the molecular weight of a • Example #1 - Al2(SO4)3 substance

• There are: • two atoms of aluminum and the atomic weight of Al is 26.98 amu. • three atoms of sulfur and the atomic weight of S is 32.06 amu. • twelve atoms of oxygen and the atomic weight of O is 16.00 amu.

First multiply: 2 x 26.98 = 53.96 total weight of all Al in formula 3 x 32.06 = 96.18 total weight of all S in formula 12 x 16.00 = 192.00 total weight of all O in formula Then add: 53.96 + 96.18 + 192.00 = 342.14 amu. This answer, 342.14 amu, represents the molecular weight of Al2(SO4)3

Review: four steps to calculating a substance's molecular weight • Step 1: Determine how many atoms of each different element are in the formula. • Step 2: Look up the atomic weight of each element in a periodic table. • Step 3: Multiply step one times step two for each element. • Step 4: Add the results of step three together and round off as necessary.

Special Note about Hydrates • Suppose you were asked to calculate the molecular weight of CuSO4 . 5H2O • The dot DOES NOT mean multiply. • You could approach this two ways: 1.Add the atomic weights of one copper, one sulfur, nine oxygens, and ten hydrogens. 2.Add the atomic weights of one copper, one sulfur, and four oxygens. Then add the molecular weight of five H2O molecules. • The answer is 249.68 amu.

Practice Problems: • Calculate the molecular weight of: • 1) (NH4)2S • 2) Fe2O3 • 3) KClO4 • 4) SF6 • 5) (NH4)2SO4

Answers to the practice problems: *Answers rounded to Sig figs based on the periodic table used 1. 2. 3. 4. 5.

68.14 amu 159.692 amu 138.65 amu 146.05 amu 132.13 amu

Just what is a mole? • The mole is the standard method in chemistry for communicating how much of a substance is present.

Here is how the International Union of Pure and Applied Chemistry (IUPAC) defines "mole:"

The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.

Where did Avagadro’s Number come from? • Using Mass Spectroscopy data, scientists describe the number of “elementary entities” in carbon-12 as a quantity that equals 6.02 x 1023 entities which is equal to one mole. • The quantity was called Avagadro’s number in honor of his work on gases and the mole concept.

What is Avagadro’s Number again? one mole of ANYTHING contains 6.02 x 1023 entities. In other words, there are 6.02 x 1023 anythings in one mole of anything!

Examples: • One mole of donuts • One mole of molecules • One mole of • One mole of • One mole of • One mole of electrons

donuts contains 6.02 x 1023 H2O contains 6.02 x 1023 nails contains 6.02 x 1023 nails Fe contains 6.02 x 1023 atoms dogs contains 6.02 x 1023 dogs electrons contains 6.02 x 1023

• When we weigh one mole of a substance on a balance, this is called a "molar mass" and has the units g/mol (grams per mole). This idea is very critical because it is used all the time. • A molar mass is the weight in grams of one mole. • One mole contains 6.02 x 1023 entities. • Therefore, a molar mass is the mass in grams of 6.02 x 1023 entities.

How do you calculate molar mass? • YOU ALREADY KNOW HOW! • The molar mass of a substance is the molecular weight in grams. • All you need to do is calculate the molecular weight and stick the unit "g/mol" after the number and that is the molar mass for the substance in question.

Example: Calculate the molar mass of Al(NO 3)3 (1 x 26.98) + (3 x 14.007) + (9 x 16.00) = 213.00 g/mol 213.00 grams is the mass of one mole of aluminum nitrate. 213.00 grams of aluminum nitrate contains 6.02 x 1023 entities of Al(NO3)3

How to convert moles to grams: There are three steps to converting moles of a substance to grams: 1.Determine how many moles are given in the problem. 2.Calculate the molar mass of the substance. 3.Multiply step one by step two.

You can convert proportionally: • The three steps above can be expressed in the following proportion: grams of the molar mass of substance the substance  moles of the one mole substance

molar mass x grams of substance = of substance one mole x moles of substance

Or, you can convert using dimensional analysis: X moles of substance molar mass (in g) one mole = grams of substance

Converting from grams to moles: • You can convert using a proportion or dimensional analysis. • You would use the same process that was used to convert moles to grams.

Converting moles to atoms, ions, molecules, etc. You can use proportions or dimensional analysis. However, you need to realize that you cannot directly convert grams to atoms, ions, molecules, etc. Converting to ions, atoms, etc requires using Avagadro’s Number (6.02 x 1023 = 1 mole)

Example: 4 moles of sulfur = _______ atoms 4 moles

6.02 x 1023 atoms= 1 moles

= 2.41 x 1024 atoms sulfur