Lecture 3 Chem 102

# Lecture 3 Chem 102 - Chapter 3 Stoichiometry Average mass...

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Chapter 3 Stoichiometry

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Average mass of the object Objects behave as though they were all identical. 37.60 g Avg. Mass of 1 Marble = = 3.76 g / marble 10 marbles Example 1. A pile of marbles weigh 394.80 g. 10 marbles weigh 37.60 g. How many marbles are in the pile? 394.80 g = 105 marbles 3.76 g
Average atomic mass (amu) Elements occur in nature as mixtures of isotopes. Carbon (C) 12 C 98.89% 13 C 1.11% 14 C < 0.01% 98.89% of 12 amu + 1.11% of 13.0034 amu = (0.9889)(12 amu) + (0.0111)(13.0034 amu) = 12.01 amu exact number (standard) There is not a single atom with mass 12.01

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Average atomic mass (amu) There is not a single atom with mass 12.01 We can consider carbon to be composed of only one type of atom with a mass of 12.01. This enables us to count atoms of natural carbon by weighing a sample of carbon.
Atomic mass Sodium has a mass of 22.99 amu on the periodic table Weighted average mass of different sodium Isotopes (13) 22 Na, 23 Na, 24 Na Na 22.99 11 75% 35 Cl, and 25% 37 C Chlorine (Cl) as an example: mass 35.5 amu 35 Cl and 37 Cl 75*35 + .25*37 = 35.5

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Mass spectrometer
Mass spectrometer Atoms or molecules are passed into a beam of electrons. Atoms or molecules loose electrons and become ions An external magnetic field is applied to the ions. The ions travel through the field and the path of their direction is bent according to their interaction with the external field. Heavier ions are deflected less that lighter ones. Where they hit the plate is indicative of their mass.

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Exercise Example 2. An element consists of 62.60% of an isotope with mass 186.956 amu and 37.40% of an isotope with mass 184.953 amu. Calculate the average atomic mass and identify the element .
The Mole (mol) Unit used to measure the amount of substance The mole is to the amount of substance as the gram is to mass. Macroscopic scale (grams) molecular scale (mole) Used for counting atoms, molecules. The mole represents a numerical value. A dozen represents 12 items (eggs or donuts) Gross is 12 dozen , 144 items 1 mole of something = 6.022 x 10 23 units of that thing (Avogadro’s number). 2 ( H 2 ) + 1 ( O 2 ) 2 ( H 2 O )

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The Mole A mole will always equal 6.022 X 10 23 atoms or molecules Atoms and molecules are very very small Represents: A specific number of particles (atoms, molecules) A definite and specified mass of a substance 1 mole C = 6.022 x 10 23 C atoms = 12.01 g C Defined as : The number equal to the number of carbon atoms in exactly 12 grams of pure 12 C. The mass of 1 mol of an element = the atomic mass in grams
The Mole 2 ( H 2 ) + 1 ( O 2 ) 2 ( H 2 O ) 2 moles of H 2 molecules react with 1 mole of O 2 molecules to produce 2 moles of H 2 O molecules Each reaction has a fixed Mole Ratio that can be used for calculations The Mole: A unit representing a very big number 1 mole eggs = 6.02 X 10 23 eggs

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Example 3. Calculate the number of iron atoms in a 4.48 mole sample of iron. Example 4. Calculate the number of copper
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## This note was uploaded on 04/21/2011 for the course CHEM 102 taught by Professor Peterpastos during the Spring '08 term at CUNY Hunter.

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Lecture 3 Chem 102 - Chapter 3 Stoichiometry Average mass...

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