Go over Ch. 9 Test
Go over HW
Section 6.1
Atoms and Moles
B. Atomic Masses: Counting Atoms by Weighing
• Atoms have very tiny masses so scientists made a unit
to avoid using very small numbers.
1 atomic mass unit (amu) = 1.66 ×10-24 g
• The average atomic mass for an element is the weighted
average of the masses of all the isotopes of an element.
Section 6.1
Atoms and Moles
C. The Mole
• One mole of anything contains 6.022 x 1023 units of that
substance.
– Avogadro’s number is 6.022 x 1023.
• The mole is defined as the number of atoms in
exactly 12g of carbon-12.
Section 6.1
Atoms and Moles
Do standard 3e (front left only) for review
Section 6.1
Atoms and Moles
A. Molar Mass
• A compound is a collection of atoms bonded together.
•
The molar mass of a compound is obtained by summing the masses of the
component atoms.
Section 6.1
Atoms and Moles
B. Percent Composition of Compounds
• Percent composition consists of the mass percent of
each element in a compound:
Mass percent =
mass of a given element in 1 mol of compound
×100%
mass of 1 mol of compound
Section 6.1
Atoms and Moles
B. Calculation of Empirical Formulas
Section 6.1
Atoms and Moles
Find the Empirical formula
• 25.95% Nitrogen
• 74.06% Oxygen
Section 6.1
Atoms and Moles
B. Mole-mole Relationships
• A balanced equation can predict the moles of product that a
given number of moles of reactants will yield.
•
The mole ratio allows us to convert from moles of one substance in a balanced
equation to moles of a second substance in the equation.
Section 6.1
Atoms and Moles
9.2 B. Mass Calculations Using Scientific Notation
• Stoichiometry is the process of using a balanced
chemical equation to determine the relative masses of
reactants and products involved in a reaction.
– Scientific notation can be used for the masses of any
substance in a chemical equation.
• To solve: gA -> mol A -> mol B -> g B
• To convert between moles and grams we use the molar
masses of the substance. To convert between moles we
use the mole ration from the balanced equation.
Temperature

7a
What is
temperature?
Temperature is a measure of the average
kinetic energy of the molecules (/atoms)
of a substance.
◦ In a hot sample, the molecules are moving
much faster than in a cold sample.
Heat

What happens when
you heat up a
substance?
Heat is energy transferred from
molecules at a higher temperature to
molecules at a lower temperature.
Heat Flow Problems

To calculate heat, use the following formula:
Always given to you!
◦ Energy = mass * specific heat * temperature change
Latent Heat

7d cont
Latent (hidden) heat is energy added to a
substance that doesn’t change the
temperature.
◦ Instead, it is used to change the phase
solid  liquid
liquid  gas
What is latent heat and
what is it used for?
Heat added here doesn’t
change the temperature of the
substance, it is causing the
phase change.
ΔH – Heat of Reaction

7b cont
Endothermic
◦ The products have more energy than the
reactants.
◦ Heat must be put into the reaction, so ΔH is
positive.

Exothermic
◦ The products have less energy than the
reactants.
◦ Heat must be released from the reaction, so
ΔH is negative.
Summary
What is temperature
 What happens when you heat a substance?
 Describe endo/exothermic reactions.
 What is ΔH for endo/exothermic reactions?
 Where does the energy go during phase
changes?
 Be able to calculate heat problems.
 What is latent heat used for?
