Transcript Chapter 8
Chemical Composition
8.1 Counting by Weighing
Which method is more effective?
Counting jelly beans vs. weighing jelly bean
total mass
Average mass = ------------------
number of samples
8.2 Atomic Mass: Counting Atoms by
Weighing
Atomic mass unit (amu): unit of mass for an atom
1 amu = 1.66 x 10-24 g
Example
Calculate the mass in amu of a sample of
aluminum contains 75 atoms
Carbon contains 62 atoms
Calculate the number of
sodium atoms present in a sample that has 1172.49 amu
Copper atoms present in a sample that has a mass of 1779.4
amu
8.3 The Mole
SI unit for an amount of substance
Countable units
Can be obtained from a balanced equation
E.g C(s) + O2(g) CO2(g)
1 atom of C reacts with 1 molecule O2 to yield 1 molecule of
CO2
OR
1 mol of C reacts with 1 mole O2 to yield 1 mole CO2
8.3 The Mole
Described the number of objects present in a dozen
Based on C-12
1 mol of anything = 6.02 x 1023 units of that substance
(atoms, molecules, particles, dollars etc..)
Avogadro’s number
1 mol of an atom = atomic weight of an atom in grams
Use as a conversion factor
E.g Atomic weight of Na = 22.99 amu
1 mol Na = 22.99 g Na
Steps in determination the
mole
Determine the final unit
Determine the atomic mass of an atom
Write out an equivalence
Express equivalence as a conversion factor
Example
Calculate the number moles of atoms and the number of
atoms in
2.50 g sample of calcium
57.7 g sample of sulfur
8.4 Molar Mass
Like density, it’s a ratio of two numbers.
Mass per one mole of substance
Unit => g/mol
E.g if 0.400 mol of a compound has a mass of 17.6 g, what is
the molar mass of the compound?
Replaced with molecular weight because molar mass
described the concept more accurately
Formula weight is used for ionic compound
Example
What is the molar mass of CH4?
Calculate molar mass of sulfur dioxide, a gas produced
when sulfur containing fuels are burned.
Calculate the molar mass for each of the substance below
(record to two decimal places)
Water
Ammonia
Barium hydroxide
Calculating the Mass from Moles
Recall
Molar mass is the mass per one mole of a substance
1 mol of substance = # g
Use as a conversion factor
E.g
if the molar mass of NaCl = 58.44 g/mol
Example
Calcium carbonate (also called as calcite), is the principal
mineral found in limestone, marble, chalk, pearls and the
shells of marine animals such as clams
Calculate the molar mass of calcium carbonate
A certain sample of calcium carbonate contains 4.86 mol.
What is the mass in grams of this sample?
Example
Calculate the mass of 4.85 mol of acetic acid, HC2H3O2.
Vinegar is a dilute solution of acetic acid
Calculate the molar mass of sodium sulfate. A sample of
sodium sulfate with a mass of 300.0 g represents what
number mole of sodium sulfate?
How many water molecules are in a 10.0g sample of water?
8.5 Percent Composition
Percent composition – the mass present of each element
present in a compound
% composition =
To calculate the percent composition (percentage composition)
of a compound
Calculate the molar mass of the compound
Calculate the total mass of each element present in the formula of the
compound
Calculate the percent composition (percentage composition):
% by weight (mass) of element = (total mass of element present ÷
molecular mass) x 100
Example
Carvone is a substance that occurs in two forms, both
of which have the same molecular formula (C10H14O)
and molar mass. One type of carvone gives caraway
seeds their characteristic smell; the other is responsible
for the smell of spearmint oil. Calculate the mass of
percent of each element in carvone.
Determine the mass percent of each element in sulfuric
acid.
8.6 Formula of Compounds
Formula of a compound represents the relative numbers of
various types of atoms present.
E.g CO2, H2O etc..
Empirical formula: simplest formula
Molecular formula: the actual formula of a compound
8.7 Calculation of Empirical Formula
In each case below, the molecular formula for a
compound is given. Determine the empirical formula
for each compound
C6H6. This is the molecular for benzene; a liquid commonly
used in industry as a starting material for many important
products
H2O2. This is hydrogen peroxide, a substance commonly
diluted with water and used as a disinfectant
CCl4. This is carbon tetrachloride, an organic solvent
8.7 Calculation of Empirical Formula
Step 1: Obtain the mass of each element present (in grams)
Step 2: Determine the number of moles of each type of atom
present
Step 3: Divide the number of mole of each element by the
smallest number of moles to convert the smallest number to 1.
If all of the numbers so obtained are integers, these are the
subscripts in the empirical formula. If one or more of these
number are not integers, go on to step 4
Step 4: Multiply the numbers you derived in step 3 by the
smallest integer that will convert all of them to whole numbers.
This set of whole numbers represents the subscripts in the
empirical formula
Example
In a lab experiment it was observed that 0.6884 g of lead
combines with 0.2356 g of chlorine to form a binary compound.
Calculate the empirical formula of this compound
When a 2.00 g sample of iron metal is heated in air, it reacts with
oxygen to achieve a final mass of 2.573 g. Determine the
empirical formula for this iron oxide
Calculating Empirical Formula from
Percent Composition
Cisplatin, the common name for a platinum compound that
is used to treat cancerous tumors, has the composition
(mass percent) 65.02% platinum, 9.34 % nitrogen, 2.02%
hydrogen, and 23.63% chlorine. Calculate the empirical
formula for csiplatin
8.8 Calculation of Molecular Formulas
Molecular formula – gives the actual numbers of
atoms in a molecule
Molecular formula = n x empirical formula
may be the same as the empirical formula
a multiple of the empirical formula (or # of empirical unit) is
defined:
Multiple ( # of empirical unit) or
n=
Example
A compound used as an additive for gasoline to help
prevent engine knock shows the following percent
composition
71.65 % Cl
24.27 % C
4.07 % H
The molar mass is known to be 98.96 g/mol. Determine
the empirical formula and the molecular formula for
this compound
A compound containing carbon, hydrogen, and oxygen
is found to be 40.00% C and 6.700% by mass. The
molar mass of this compound is between 115 g/mol
and 125 g/mol. Determine the molecular formula for
this compound