Mole Concept. Atomic Mass §The mass of a single atom is too small to measure on a balance. 1.673 x 10 -24 g §Using a mass spectrometer the mass of one.

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Presentation transcript:

Mole Concept

Atomic Mass §The mass of a single atom is too small to measure on a balance x g §Using a mass spectrometer the mass of one hydrogen atom was measured to be x g. §Numbers of this size are too small for practical use. §A system of relative atomic masses was devised to express the masses of elements using simple numbers. §Using this system, the number of atoms of each element present is not expressed in the name.

§The system of relative atomic mass is based on the mass of the isotope of carbon that contains 6 neutrons: was assigned a value of exactly 12 atomic mass units x g 1 amu = mass of = x g In the table of atomic masses the masses of all elements are based on the mass of The atomic masses of many elements deviate considerably from whole numbers. Rb = amu Mg = amu

§The atomic mass of an element is the average relative mass of the isotopes of that element compared to which is exactly amu. §Magnesium is times heavier than §The determination of the atomic mass for Mg is shown below §Atomic masses of each element could be determined in a similar manner.

Molar Mass §The atomic mass of an element in grams contains Avogadro's number of atoms and is defined as the molar mass of that element. §To determine molar mass, we change the units of atomic mass (found in the periodic table) from atomic mass units (amu) to grams (g). §One gram-mole of an element contains Avogadro's number of atoms of that element. §One gram-mole of a compound contains Avogadro's number of molecules of that compound.

Calculation of Molecular Mass §The molecular mass of any molecule or formula unit is equal to the sum of the atomics masses of all of its atoms. (Recall a molecule consists of two or more atoms bonded together.) §Example: Calculate the molecular mass of C 2 H 6 O 2 x C =2 x = amu 6 x H =6 x 1.01 = 6.06 amu 1 x O =1 x = amu amu Therefore, molar mass of C 2 H 6 O is amu

Example: Calculate the formula mass of LiClO 4 1 x Li = 1 x 6.94 = 6.94 amu 1 x Cl = 1 x = amu 4 x O = 4 x = amu amu molecular or formula mass of LiClO 4 is amu §Example: Calculate the molecular mass of (NH 4 ) 3 PO 4 3 x N = 3 x = x H = 12 x 1.01 = x P = 1 x = x O = 4 x = amu molecular or formula mass of (NH 4 ) 3 PO 4 is amu Molar mass and formula mass are the same

The Mole §Single atoms are too small to weigh and too small to count. §Chemists require a unit which can count large numbers of atoms and large numbers of molecules. §If the numbers of atoms and molecules present are known then their total weights can be easily determined. §Chemists have chosen the term mole as the unit for counting atoms and molecules. Just like a dozen of is 12 objects and a pair is two objects, a mole of any object is x objects.

§1 mole objects = x objects §The number x is a number used quite frequently in chemistry and it is called Avogadro's Number or N A 1 mole atoms = x atoms 1 mole ions = x ions 1 mole molecules = x molecules 1 mole of pizzas = x pizzas (YUMMM!)

Recall that an “amu” was assigned the mass of x g based on 12 C. §If you have a group of items and you know the fractional weight of each item, can you determine the number of items in the group? l Eg. - 1 egg is 1/12 th of a group, then there must be 12 eggs in the group ( 1/12 x 12 = 1 dozen). - 1 item is 0.5 parts of the group, then there must be 2 items in the group (0.05 x 2 = 1) -1 item is 0.01 parts of the group, then there must be 100 items in the group (0.01 x 100 = 1) -1 item is x parts of the group, then there must be x items in the group ( x x x = 1) -We call the last group of x items the “Mole”.

Molar Mass & Diatomic Molecules §In dealing with diatomic elements (H 2, O 2, N 2, F 2, Cl 2, Br 2, I 2 ) distinguish between 1 mole of atoms and 1 mole of molecules. §Examples: Calculate the molar mass of 1 mole of H atoms. 1 x H = 1 x 1.01 g = 1.01 g Calculate the molar mass of 1 mole of H 2 molecules. 2 x H = 2 x 1.01 g = 2.02 g Calculate the molar mass of 1 mole of O atoms. 1 x O = 1 x 16.0 g = 16.0 g Calculate the molar mass of 1 mole of O 2 molecules. 2 x O = 2 x 16.0 g = 32.0 g

Conversions Using the Mole

The Molar Islands Mass IslandNumber IslandMole Island Toll: Molar mass Toll: Avagadro’s number

§To convert from grams of a substance to moles of a substance (and vice versa) we have to use the molar mass of the substance. §To convert from moles of a substance into atoms (or molecules) of the substance (and vice versa) we have to use Avogadro’s number. gramsmoles molecules (or atoms) Molar mass Avogadro’s # FORBIDDE N

Moles to Grams §The molar mass of any substance is the number of grams contained in one mole of that substance. Example: What is the mass in grams of 4 moles of H 2 ?

Example How many grams of copper are contained in 2.52 moles of copper?

Example How many grams of (NH 4 ) 3 PO 4 are contained in 2.52 moles of (NH 4 ) 3 PO 4 ? ElementHow Many?Mass EachMass Contribution N H P O

Grams to Moles §Example: How many moles of benzene C 6 H 6 are present in 390 grams of benzene. ElementHow Many?Mass EachMass Contribution C61272 H616

Moles to Atoms (or Molecules) §To convert from moles to atoms (or molecules) or vice versa we have to use Avogadro’s number. §The conversion factors are as follows :

§Example: 7.00 moles of N 2 contains how many N 2 molecules? §Example: 7.00 moles of N 2 contains how many N atoms?

§Example: How many Na atoms are contained in 2.00 moles Na? §Example: How many molecules of H 2 O are present in 5.00 moles of H 2 O?

Grams to Atoms (or molecules) §To convert from grams to atoms or molecules requires: l the conversion of grams into moles first using the molar mass. l Secondly the conversion of moles to atoms (or molecules) using Avogadro’s number.

§Example: How many molecules of hydrogen chloride HCl are there in 25.0 grams of hydrogen chloride?

CONCLUSION §Use the following diagram to help with the conversions between moles, grams and particles gramsmoles molecules (or atoms) Molar mass Avogadro’s #

Better yet………. Welcome to The Molar Islands Mass IslandNumber IslandMole Island Toll: Molar mass Toll: Avagadro’s number DON’T FORGET TO PAY THE TOLLS

The End §Well, not yet §More examples to follow

Calculation of Molar Mass §The molar mass of any molecule is equal to the sum of the molar masses in grams of all of its atoms. Example: Calculate the molar mass of C 2 H 6 O

Calculation of Molar Mass §The molar mass of any molecule is equal to the sum of the molar masses in grams of all of its atoms. Example: Calculate the molar mass of C 2 H 6 O 2 x C = 2 x g = g 6 x H = 6 x 1.01 g = 6.06 g 1 x O = 1 x g = g g

Example: Calculate the molar mass of LiClO 4 Example: Calculate the molar mass of (NH 4 ) 3 PO 4

Example: Calculate the molar mass of LiClO 4 1 x Li = 1 x 6.94 g = 6.94 g 1 x Cl = 1 x g = g 4 x O = 4 x g = g g Example: Calculate the molar mass of (NH 4 ) 3 PO 4 3 x N = 3 x 14.01g = g 12 x H = 12 x 1.01 g = g 1 x P = 1 x g = g 4 x O = 4 x g = g g