Transcript The Mole - Polson 7-8

The Mole
Objectives
• Be able to write conversion factors.
• Be able to make conversions using
dimensional analysis.
Dimensional Analysis
Dimensional analysis is a method for converting
values and units in the sciences.
equality: two equal amounts
1 cup = 8 fl oz
1.61 km = 5280 feet
760 mm Hg = 101.3 kPa
conversion factor: a ratio made from an equality;
always = 1
Dimensional Analysis Problems
Easy…
How tall is Mt. Everest
in km if its elevation is
29035 ft?
More difficult…
How much does it cost
to drive 1400 miles if your
car gets 32 miles per gallon
and gas costs $3.49 per
gallon?
Objectives
• Understand and explain the concept of the
mole.
• Be able to determine the molar mass of a
substance.
The Mole
What are some terms that represent a specific number?
pair = 2
dozen = 12
gross = 144
google = 10100
mole = 6.02 x 1023
Why this number??
Avogadro’s
Number
1 atom Cu = 63.5 u
1 molecule H2O = 18.0 u
1 mole of Cu atoms = 63.5 g
1 mole of H2O = 18.0 g
Different masses, but the same number of particles!
Molar Mass
molar mass: mass in grams of 1 mole of any
substance (usually written as # g/mol)
Just “add up the masses” to find the molar mass
of the following
Se
BaCl2
Fe2(CO3)3
NaC2H3O2∙3H2O
Why Do We Use Moles?
• Suppose you want to react Cu and S to make Cu2S.
The ratio of atoms is 2:1.
• You can’t simply react a 2:1 ratio of grams
• You must react a 2:1 ratio of moles
• For example, 0.50 mol Cu and 0.25 mol S.
• You must calculate how many grams of Cu and S are
needed from these amounts.
Objectives
• Be able to convert from moles to grams or
from grams to moles for a given amount of
substance.
• Be able to calculate and measure needed
amounts of substance in the lab.
Converting g ↔ mol
How many moles of copper are in a 12.4 g sample?
What is the mass of 0.0472 mol S?
Converting g ↔ mol
What is the mass of 0.0500 mol CaCl2?
How many moles are in 4.83 g Sr(NO3)2?
Objectives
• Be able to calculate the percentage
composition for a compound.
• Be able to determine the empirical formula of
a compound from mass data.
Percentage Composition
percentage composition: shows mass percentage
of each element in a chemical compound
• find molar mass
• divide mass of each element by total molar mass
What is the percentage composition of K2SO4?
Empirical Formulas
empirical formula: a chemical formula determined from
experimental data; shows the lowest whole-number
ratio of the atoms
• convert masses to moles
• find lowest whole-number ratio—this is the formula
Example: 1.587 g Cu reacts with 0.402 g S to form
a compound. What is the empirical formula (and
stock name) of the compound?
Empirical Formulas
0.56 g chromium is placed in a crucible and burned.
The mass of the resulting chromium oxide is 0.82 g.
What is the empirical formula?
Empirical Formulas
What is the empirical formula of “red
lead,” a compound that contains
90.7% lead and 9.33% oxygen?
• just use grams for % values
Objectives
• Be able to explain the difference between an
empirical formula and a molecular formula.
• Be able to determine the molecular formula of
a compound when given the molar mass and
the empirical formula.
Molecular Formulas
molecular formula: shows the actual # of
the atoms in a molecule
empirical formula: lowest whole-number ratio
Example: butane
molecular = C4H10
empirical = C2H5
The empirical formula of a compound is CH2O.
The actual mass of its molecules is 180.0 g/mol.
What is the molecular formula of the compound?