Download Chapter 3 Stoichiometry: Chemical Calculations

Honors Chemistry
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Unit 3 – Mass Relations in Chemistry; Stoichiometry
Atomic Masses
Atomic mass – (atomic weight) –
Atomic mass units – (amu) –
Carbon-12 scale –
Atomic Masses and Isotopic Abundances:
Mass spectrometer –
Isotopic abundances –
Atomic Mass Calculations:
Atomic mass Y = (atomic mass Y1) x %Y1 + (atomic mass Y2) x %Y2 +…
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Example 1: Bromine is a red-orange liquid with an average atomic mass of
79.90 amu. Its name is derived from the Greek word bromos, which means
stench. It has two naturally occurring isotopes: Br-79 (78.92 amu) and Br-81
(80.92 amu). What is the abundance of the heavier isotope?
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Masses of Individual Atoms; Avogadro’s Number:
Avogadro’s Number –
For Example:
6.02x1023 H atoms in 1.008 grams of H (atomic mass of H = 1.008)
Example 2: Consider titanium (Ti). Taking Avogadro’s Number to be
6.02x1023, calculate:
a) the mass of a titanium atom
b) the number of atoms in a 10.0g sample
c) the number of protons in 0.1500 pounds of titanium
The Mole
mole –
specialized units – the correct name for a particle of a substance based
on the type of matter
a. atom –
2
b. ion –
c. molecule –
d. formula unit -
molar mass – (MM)-
Calculating molar mass:
1. find the mass of the element on the periodic table
2. multiply by the number of atoms of that element in the formula
(distribute parenthesis)
3. sum the relative masses of the individual elements
Example 3: Determine the molar masses of the following substances.
Aluminum
CaSO4
(NH4)3P
The Significance of the Mole:
In the laboratory, substances are weighted on balances in units of grams
The mole allows us to relate the number of grams of a substance to the
number of atoms or molecules of a substance
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Mole – Gram Conversions:
Molar mass can be used like any other conversion factor:
Molar mass (g) = 1 mole
Example 4: acetylsalicylic acid, C9H8O4, is the active ingredient in aspirin.
A) What is the mass in grams of 0.509 moles of acetylsalicylic acid?
B) A 1.00 g sample of aspirin contains 75.2% by mass of C9H8O4. How
many moles of acetylsalicylic acid are in the sample?
C) How many molecules of C9H8O4 are there in 12.00g of acetylsalicylic
acid? How many carbon atoms?
Mass Relations in Chemical Formulas
Percent composition from formula:
Percent composition –
Part x 100 = % composition
Whole
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2 Types of % Comp. problems:
1.
2.
given data as masses of elements, etc.
a. use the masses of each element and the total mass of the
compound
given the chemical formula of the compound
a. use the relative molar masses of each element and the molar
mass of the compound
Example 5: Metallic iron is most often extracted from hematite ore, which
consists of iron (III) oxide mixed with impurities such as silicon dioxide, SiO2.
A) What are the mass percents of iron and oxygen in iron (III) oxide?
B) How many grams of iron can be extracted from 1.00 kg of Fe 2O3?
C) How many metric tons of hematite ore, 66.4% Fe 2O3, must be
processed to produce 1.00 kg of iron? (1 metric ton = 1,000kg)
Subscripts:
1.
2.
represent the atom ratio in a compound
represent the mole ratio in a compound
Diatomic Elements:
Elements that due to there chemical reactivity exist only as
molecules of 2 atoms in nature.
7 diatomic elements:
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Simplest Formula from Chemical Analysis (Empirical Formula):
Simplest formula – (empirical formula) –
Calculating the empirical formula:
1. can be determined from masses of the individual elements or
the % composition of the elements in a compound
2. if %’s are given consider the sample to be of 100 grams and so
the %’s become the masses in grams:
For example: 25.6% = 25.6 g
3. convert the mass of each element to moles
4. divide each number of moles by the smallest number of moles
of all of the answers to #3
5. * If the answers to #4 are whole numbers, these are the
subscripts in the empirical formula.
* If any of the answers to #4 is not a whole number, convert all
answers to a common fraction. Multiply each fraction by the
denominator resulting in a whole number and these are the
subscripts in the empirical formula.
Common Fractions:
0.25
0.33
0.50
0.66
0.75
Example 6: A 25.00g sample of an orange compound contains 6.64g of
potassium, 8.84g of chromium, and 9.52g of oxygen. Find the empirical formula.
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Example 7: The compound that gives vinegar its sour taste is acetic acid, which
contains the elements carbon, hydrogen, and oxygen. When 5.00g of acetic
acid is analyzed it is found to contain 2.00g of carbon, 0.336g of hydrogen, and
2.66g of oxygen. What is the empirical formula of acetic acid?
Molecular Formula:
Molecular formula –
Calculating the molecular formula:
1.
find the molar mass of the empirical formula
2.
divide the molecular molar mass (given in the question) by
the empirical molar mass
MMM
EMM
3.
multiply each subscript in the empirical formula by the
answer to #2, these are the subscripts for the molecular
formula
Example 8: The molecular molar mass of acetic acid is 60.0 g/mole. According
to the empirical formula found in example 3.6, what is the molecular formula of
acetic acid?
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