Transcript Using the Mole

Using the Mole
Avogadro’s Number
Mole Conversions
Determining Chemical Formulas
Counting Particles
Chemists need a way to accurately count
atoms, ions, formula units and molecules.
All of these are extremely small.
Chemist created the unit called the mole to
express the enormous number of particles in
a sample.
What is a mole?
SI base unit for amount of a substance
Avogadro’s Number of particles (NA)
602 200 000 000 000 000 000 000
particles
6.022 x 1023 particles = 1 mol of particles
Molar Mass = Mass of 1 mole of
 Atoms
= atomic mass (for elements)
 Molecules = molecular mass (for
covalent compounds)
 Formulas units = formula mass (for
ionic compounds)
 Ions = formula mass (for individual
ions)
Molar Mass of an Element
H = 1.00794
C = 12.0107
Fe = 55.845
Cu = 63.546
The molar mass for an element is
just its average atomic mass,
which you can find in the
PERIODIC TABLE!
Molar Mass of a Compound
called formula mass for ionic
compounds
Called molecular mass for
covalent compounds
FeCl2
H2O
CO2
Calculating Molar Mass
1. Use the formula for the compound
2. Look up molar mass of each element in the
compound
3. Multiply the molar mass for each element
by how many atoms of each element are in
the compound
4. Add the “adjusted” molar mass for each
element together What are the units for
molar mass?
Mole Conversions:
Your conversion factor is
always the molar mass
Example 1: How many moles are in 48.0 g of C ?
Given
48.0 g C X
1 mol C = 4.00 mol C
12.0 g C
Mole Conversions:
Example 2: How many grams are in 6.00 mol of
HCl ?
Remember: What is a mole?
The mole is a counting unit
We can count atoms, molecules, formula
units, or ions using moles
Conversion Factor
1 mol of units = 6.022 x 1023 units
Mole Conversions:
Example 3: How many atoms are in
3.00 mol of N?
Mole Conversions:
Example 4: How many molecules are in
3.2 moles of H20?
Mole Conversions:
Example 5: How many formula units are in
1.5 mol of K2(CO3)
How many potassium ions are in 1.5 mol of
that compound?
☺
2-step mole calculations
What is the mass of 3.8 x 1034 Fe atoms?
2-step mole calculations
How many molecules are in 176 g of water?
% Composition
FeCl3
Divide mass of each
element by total formula
mass and multiply by 100
% Composition
(NH4)3(PO4)
Divide mass of each
element by total formula
mass and multiply by 100
Determining Empirical Formula
If the identities of the elements in a
compound are known, then the empirical
formula can be determined from:

Masses
Change grams to moles
 Determine the mole ratio


Percentage Composition (assume 100 g sample)
Change grams to moles
 Determine the mole ratio

What is the empirical formula of a compound
containing 0.90 g of Ca and 1.60 g of Cl?
What is the empirical formula of a compound
that is 66.0% Ca and 34.0% P?
Molecular Formula
In order to determine
the molecular formula,
you need the empirical
formula and the molar
mass
Find the empirical formula if not
given
Divide molecular mass by empirical
formula mass
Multiply each atom by factor
Determine the molecular formula of a
compound if the empirical formula is CH20
and the molecular mass is 180.0 g/mol.
Find the empirical formula mass:
C – 1 x 12.0 = 12.0
180.0
30.0
=6
H – 2 x 1.0 = 2.0
O – 1 x 16.0 = 16.0
Divide the MM by the EFM
30.0
Multiply all subscripts of the EF by 6
C6H12O6
Finding the formula for a hydrate:
Crystals that contain water molecules

CuSO4 • 5 H20
Copper sulfate pentahydrate
Water is lost as a hydrate is heated to form
an anhydrous salt

CuSO4
To determine the formula of a hydrate:
Convert grams to moles
 Divide by the lowest to find the coefficient for
water (round to the nearest whole number)

Determine the formula for a hydrate that
is 76.9 % La2(CO3)3 and 23.1 % H20.