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The Mole
I Chemical measurements
A. atomic mass and formula mass
1. amu used to measure the mass of an atom or compound
2. measured relative to the mass of C-12 , a standard
3. formula mass - sum of the atomic masses of all atoms in a compound
B. what is a mole?
1. amu is the mass of one atom or molecule
2. this quantity is not practical
3. chemists need the mass in grams
4. H O atom ratio
1 amu 16 amu 1:1
1 g 16 g ?:? <-- chemists needed to know
5. experimentally determined to be 6.02x1023 (based on C-12)
6. called a mole (mol)
7. the mass in grams of 1 mole of any substance is numerically equal to its atomic mass or formula mass
8. the chemist's dozen
C. what's in a mole
1. atoms - 1 mole of an element (ex. Fe) = 6.02x1023 Fe atoms
2. molecules - 1 mole of H2O contains:
a. 1 mole (6.02x1023) of H2O molecules
b. 3 moles of atoms
c. 2 moles of hydrogen atoms
d. 1 mole of oxygen
3. formula units - 1mole of CaCl2
a. 1 mole of CaCl2 formula units
b. 3 moles of ions
c. 1 mole of Ca ions
d. 2 mole of Cl ions
D. Avogadro's number
1. 6.02x1023
2. symbol -N
3. named after Amadeo Avogadro Italian chemist and physicist
4. an enormous number - 6.02x1023 grains of rice would cover the land mass of Earth to a depth of 75 meters
E. molar mass
1. the mass of 1 mole of a substance
2. molar mass determined from its atomic mass or formula mass
II Mole Conversion
A. mass and mole
1. mole: measures both mass, number, and volume of particles
2. central unit in converting from one measurement to another
3.
a. grams------------------------------> moles
molar mass
g x 1mol = mol
molar mass (g)
b. molar mass
moles -----------------------------> grams
mol x molar mass(g) = grams
1 mol
B. particles and moles
1. do not use molar mass but rather Avogadro's number: 6.02x1023
2.
a. particles ----------------------------> moles
Avogadro's number
particles x 1 mol = moles
6.02x1023 (particles)
b. Avogadro's number
mol --------------------------------> particles
mol x 6.02x1023 (particles) = particles
1 mol
C. multistep conversions
1. Avogadro's number
mass ---------------------> mol --------------------> particles
molar mass
g x 1mol x 6.02x1023 (particles) = particles
molar mass (g) 1 mol
2. molar mass
particles -------------------> mol --------------------> mass
Avogadro's number
particles x 1 mol x molar mass(g) = grams
6.02x1023 (particles) 1 mol
D. moles and gases
1. 1 mole of any gas at the 0 degrees C and 1 atmosphere of pressure (standard conditions) occupies 22.4 L of volume
2. 22.4 L called the molar volume
3. used to convert between the volume of gas and moles, particles, and mass of a gas at STP
4.
mass particles
mol
1 mol or 22.4 L <--- conversion factor
22.4 L 1 mol
volume
III Empirical and Molecular Formulas
A. percentage composition
1. the mass of each element in a compound compared to the entire mass of the compound and multiplied by 100%
2. indicates the mass made up by each element in the compound
3. determined in two ways
a. chemical formula
1. calculate the molar mass using the chemical formula of the compound CaCl2
Ca 1 x 40.1 µ = 40.1µ
Cl 2 x 35.5 µ = 71.0µ
111. 1µ molar mass
2. divide the individual mass of each element by the molar mass
Ca 40.1µ = .361
111.1µ
Cl 71.1µ = .640
111.1µ
3. multiply each calculation by 100%
Ca .361 x 100% = 36.1%
Cl .640 x 100% = 64.0%
b. experimental analysis
1. measure the mass of a compound sample
2. chemically decompose the compound into its individual
elements
3. measure the mass of each element
4. divide each individual mass by the sample mass and
multiple by 100%
B. determining empirical formula
1. empirical formula: simplest whole-number ratio of the atoms of the elements in a compound
2. use percent composition
a. compound Ca 36.1% Cl 64.1%
b. convert % --> grams
1. assume a 100g sample
2. allows you to change label from % to grams
3. Ca 36.1% ---> 36.1g
Cl 64.1% ---> 64.1g
c. convert grams to mol
Ca 36.1g x 1mol = .900 mol
40.1g
Cl 64.1g x 1mol = 1.81mol
35.5 g
d. divide each mol value by the smallest mol value
Ca .900mol = 1
900mol <---- mol ratio
Cl 1.80mol = 2
.900mol
e. combine the mol ratio with the symbol CaCl2
C. determining molecular formula
1. molecular formula: the actual number of atoms of each element in a molecular compound
2. always a whole-number multiple of the empirical formula
3. determined by dividing the molar mass by the empirical formula mass
a. molar mass is determined experimentally
b. empirical formula mass determined just as described above for molar mass (section IIIa)
4. answer indicates how many times more atoms of each element there are in the molecular formula
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