Chemistry – Dr. May Notes
Molar Relationships
How many is a mole?
Avogadro’s number is 6.02 x 1023. That is the number of molecules or atoms (in
the case of elements) in one mole of anything.
Mass of a Mole of Atoms
The mass of one mole of any element is the molar mass of that element. One mole
of sodium has a mass of 23 grams and one mole of lead has a mass of 207.2 grams. An
atom of lead would weigh 9 times as much as at atom of sodium.
The molar mass of an element is the same number as the atomic mass units (amu)
of that element. A mole of aluminum is 26.98 grams. The atomic mass of one aluminum
atom is 26.98 amu.
Aluminum and lead have the same number of atoms in a mole but have different
molar masses. An atom of lead would weigh 7.7 times more than an atom of aluminum.
Molar Conversions of Elements
How many moles of carbon are in 26 grams of carbon?
Moles of Carbon = Grams of Carbon ÷ Molar Mass of Carbon
Moles of Carbon = 26 grams of Carbon = 2.2 moles
12.0 grams/mole
Molar Mass ≈ Atomic Mass ≈ Atomic Weight ≈ Molecular Weight
Molar Conversions of Compounds
Count atoms and add atomic masses  Molecular weight
Concentration
Concentration refers to the amount of a substance (solute) is in solution (solvent).
Solutions can be classified as concentrated or dilute.
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The term “ppm”
1% is 1 part per hundred
0.1% is 1 part per thousand
0.01% is 1 part per ten thousand
0.001% is 1 part per hundred thousand
0.0001% is 1 part per million
Molarity
Molarity is the concentration of a solution in moles per liter.
A 1M solution of NaCl means that 1 liter of a NaCl solution contains one mole or
(23.0 + 35.4) = 58.4 grams of NaCl.
In a like manner, 1 liter of a 0.1 M solution of sodium chloride contains 5.84
grams of NaCl (0.1 mole).
Or, 100 ml (0.10 liters) of a 1 M solution of NaCl contains 5.84 grams of NaCl
(0.1 mole).
Or, 100 ml of a 0.1 M solution of NaCl contains 0.584 grams of NaCl which is
0.01 moles.
Preparing Solutions
To prepare a 1M solution of NaCl, place 58.4 grams of NaCl in a 1-liter volumetric
flask and add water to the 1-liter mark.
Percent Composition
Percent means “parts per hundred”. If a 36-gram sample of iron oxide contains 28
grams of iron, that is 78% iron by weight.
Determining Empirical Formulas
Isopentyl acetate is: CH3COOCH2CH2CH(CH3)2 (Structural Formula)
C7H14O2 is the Molecular Formula
The empirical formula is defined as the smallest whole-number ratio of various
types of atoms in a compound. Therefore
C7H14O2 is also the Empirical Formula
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Steps in determining Empirical formulas:
1. Find the mass of each element in the sample of the compound.
2. Divide the mass of each element by its molar mass to find the number of
moles of each element.
3. Divide the number of moles of each element by the smallest number of moles
to give the ratio of atoms.
Determining Molecular Formulas
A molecular formula is the formula for a molecular compound.
The empirical formula gives only the whole number ratio of the atoms. With
isopentyl acetate they are the same, but with others they are different:
With C8H18 (octane), C8H18 is the molecular formula, C4H9 is the empirical
formula, and CH3(CH2)6CH3 is the structural formula.
Lets determine the empirical and molecular formula of an organic compound that
has a molecular weight of 180 (one mole = 180 grams). It has 7.20 grams of carbon, 1.20
grams of hydrogen, and 9.60 grams of oxygen.
7.20 grams of carbon = 0.600 moles of carbon (7.20/12.0)
1.20 grams of hydrogen = 1.2 moles of hydrogen (1.2/1.0)
9.60 grams of oxygen = 0.600 moles of oxygen (9.60/16.0)
Divide each of the moles by the smallest number of moles to give:
1.00 carbon, 2.0 hydrogen, and 1.00 oxygen for an empirical formula of CH2O with an
empirical formula mass of 12 + 2 + 16 = 30.
Then 180  30 = 6 and (CH2O)6 = C6H12O6 (sugar).
Significant Digits
Number
Digits to Count
Example
Significant Digits
Non-zero digits
Leading zeros
Captive zeros
Trailing zeros
All
None
All
Count only if
number contains a
decimal point
Scientific Notation
All
3279
0.0045
5.007
100
100.
100.0
0.0100
1.7 x 10–4
1.30 x 10–3
4
2
4
1
3
4
3
2
3
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When doing addition and subtraction, the number with the lowest decimal value
determines the number of significant digits in your answer. 3.75 + 4.1 = 7.85 which
becomes 7.8 because 4.1 has two significant digits. Likewise with multiplication (or
division) the significant digits in the answer are determined by the number with the
lowest decimal value. 0.59 x 1.01 = 0.584158451 and is rounded off to 0.58 because 0.59
has two significant digits.
When rounding fives, round down if the digit before the 5 is even and round up if
the digit before the five is odd. Thus, 7.75 becomes 7.8 and 7.85 becomes 7.8.
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