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Chapter 3

CHM 111 Chapter Notes - Chapter 3: Chemical Formula, Chemical Equation, Carbon-12


Department
Chemistry
Course Code
CHM 111
Professor
Acevedo Orlando
Chapter
3

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CHAPTER 3: STOICHIOMETRY
Atomic Masses
A. Average mass is found by taking the average of all of an element’s isotopes masses,
taking into account the percentage of which appear naturally on earth
a. Ex: Most of the carbon found naturally is carbon-12 (98.8%) and a small
percentage is carbon-13 (1.1%). Carbon-14 has a negligible percentage. So to find
the average mass you do
i. (.0988 x 12amu) + (.0111 x 13.003355 amu)= 12.01 amu
The Mole
A. Used in counting atoms
B. Mole: the number equal to the number of carbon atoms in exactly 12 grams of pure
carbon-12
C. Avogadro’s Number: One mole of something consists of 6.022 x 10^23 units of the
substance
a. 6.022 x 10^23 atoms in one mole
D. A sample of a natural element with a mass equal to the element’s atomic mass expressed
in grams contains 1 mole of atoms
E. 1 mole= 1 gram
Molar Mass
A. The mass in grams of 1 mole of a compound
Percent Composition of Compounds
A. Mass Percent: found by comparing the mass of one element of a compound to the total
mass of the compound and multiplying by 100%
Determining the Formula of a Compound
A. Empirical Formula: the simplest whole-number ratio of the various types of atoms in a
compound
B. Determining an empirical formula
a. Base calculation on 100 grams. Each percent will represent the mass in grams of
that element in the compound
b. Determine number of moles of each element present in 100 grams using the
atomic weights
c. Divide each value of the number of moles by the smallest of the values. The
results should be a whole number that represents the subscripts of the elements
d. If they are not whole numbers, multiply each number by an integer
C. Determining molecular formula
a. Method 1
i. Obtain the empirical formula
ii. Compute the empirical formula mass
iii. Calculate the ratio of molar mass: empirical formula mass
iv. Multiply the resulting integer by the subscripts
b. Method 2
i. Use the mass percents and the molar mass to determine the mass of each
element present in 1 mole of a compound
ii. Determine the number of moles of each element present in 1 mole of the
compound
iii. Integers from the previous steps represent the subscripts in formula
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