1 the mole. 2 atoms and molecules are very, very small, but we still need to “count” them
TRANSCRIPT
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The MOLE
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Atoms and molecules are very, very small, but we still
need to “count” them.
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The molemole is a
quantity; it is used to
“count” atoms and molecules.
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How we count atoms quickly
We have words that represent numbers that we use everyday.
FOR INSTANCE:– A dozen always = 12 items– A ream of paper always = 500 sheets of
paper– A trio always = 3 of the item.
SO IN CHEMISTRY………..
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…. we have the mole
1 mole = 6.02 x 1023 particles
AND
6.02 x 1023 is called
Avogadro’s number
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1 mole of Al atoms = 6.02 x 1023 Al
atoms1 mole H2O molecules = 6.02 x 1023 H2O
molecules1 mole jelly donuts = 6.02 x 1023 jelly
donuts1 mole of = 6.02 x 1023
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6.02 x 1023 is a VERY large
number
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602,000,000,000,000,000,000,000
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If Kevin took 1 mole of steps, each 15 cm long, he would
walk around the earth 2.26 x 1015 times, which is 2,260,000,000,000,000 or
2,260 trillion times!
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We cannot count
6.02 x 1023 Al atoms, it is just too too many.
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We can, however, measure 26.98 grams
of Al.
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26.98 grams of Al contains 6.02 x 1023 Al
atoms!
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26.98 grams of Al contains 1 mole of Al
atoms!
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The formula weight of any substance = mass of 1 mole of that substance in grams.
(26.98 grams is the formula weight of Al.)
So 26.98 grams of Aluminum is ONE mole of Aluminum.
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The formula weight of any element is the weight of an atom/molecule expressed in amu’s.
(26.98 amu is also the atomic mass of Al)
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More about amu
While amu is a valid unit for expressing weights of atoms and molecules, we are not going to be using it that often.
The reason is that it is the mass of a single unit and we are not able to use a single unit, since it is too small.
Know the definition of amu and all is well.
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The formula weight of any compound is the sum of the formula weights of all the atoms in that compound, expressed in grams.
(The formula weight of H2O = 18 grams per mole. (g / n ))
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Identical Terms The following terms on the next slide are often
interchangeable . – All can be expressed in grams or amu depending on
the application.– The term mass or weight is also often
interchangeable. Since all of our calculations will be done on Planet Earth, the gravity is constant so we can make this simplification.
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Here are the terms. Formula weight: The most general of all the
terms on this list. Atomic weight (or mass): The weight of a mole
of a single element. Molecular weight (or mass): The weight of a
mole of a single compound. Molar mass: The weight of one mole of
material. Just a rephrasing of above.
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Molar conversions: The magic formula
Mass in grams of an element or compound divided by molar mass = number of moles.
We can make this into a formula and this is without a doubt the
MOST IMPORTANT FORMULA
IN CHEMISTRY.
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The MAGIC FORMULA
The formula referred to on the earlier slide is:
Moles = Grams / Formula Weight OR
n = g / M
This is not a formula –
This is THE FORMULA !!
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Quantity conversions: Moles to number of particles
Number of atoms or molecules divided by Avogadro’s number = number of moles.
Set up a proportion !!
We can all cross-multiply
1 mole = x moles
6.02 x 1023 y particles
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PracticeFill out the following table in your notebook.
Number of particles Moles
4.56 x 1025 ___________
7.66 x 1022 ___________
_____________ .234
_________ 34.56
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Practice AnswersFill out the following table in your notebook.
Number of particles Moles
4.56 x 1025 75.75
7.66 x 1022 .127
1.41 x 1023 .234
2.08 x 1025 34.56
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Moles to Liters
If the material is a gas, we can use the fact that 1 mole of gas = 22.4 Liters to find its volume.
We should set up a proportion. 1 mole = x moles
22.4 L y Liters
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Subscripts in chemical formulas can help us represent moles!
Since 1 molecule of CO2 contains 1 atom of C and 2 atoms of O, then…
1 dozen molecules of CO2 contain 1 dozen atoms of C and 2 dozen atoms of O, and…
1 mole CO2 contains 1 mole C and 2 moles O !
This is a very useful relationship.
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Molarity
Concentration of a solution, i.e. the amount of solute that is dissolved in a given amount of solvent
number of moles of solute per liter of solution
Units = mol/L OR n / L
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Percent Composition Percent that each part of a substance is of the
whole, by mass. The whole mass is always the formula weight !
The sum of the percents must = 100 % % part = (mass of part / mass of whole) x 100 % The percent composition of CO2 is as follows…
Wt. of C = 12g Wt. of O = 16g FW = 44 g
% C = (12 / 44) x 100 % = 27.27 %
% O = ((2 x 16 ) / 44) x 100 % = 72.73 %
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Empirical Formula
Formula expressing the simplest whole number ratio of atoms in a compound
The empirical formula is often not the TRUE formula.
It is used to identify a compound in situations like forensics or learning about a new compound.
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More on Empirical Formula C6H12O6 has an empirical formula CH2O
Found by calculating the ratio of the moles of each element in the compound.
STEPS (also found on the website)– Convert each given to number of moles.
Assume a 100 gram sample if needed.
– Divide by the lowest number of moles to get a ratio.– Manipulate the values so they are whole numbers.
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Molecular Formula Determining the True or Actual Formula of the
compound. The empirical formula is needed first. The weight of the molecular formula is given.
– It will always be a multiple of the empirical formula.
The molecular formula is a multiple of the empirical formula.
CH2 is the empirical formula for all of the following: C2H4 C3H6 C4H8 C6H12 and many more