Transcript Chapter 10

The Mole
Intro to Stoichiometry
Measurements in Chemistry
 Atomic Mass: the mass of an atom of a
certain element in atomic mass units (amu).

1 amu = 1.66 x 10-24g
 Formula Mass: the sum of atomic masses of
all atoms in a compound

units are still amu
 Atomic mass and formula mass are
impractical for laboratory work.
 Why?
 1. We measure masses by grams, not amu
 2. If the atomic mass or formula mass is
converted to grams, the number is still so tiny
that it would be impossible to measure.
The Mole
 A Mole is 6.02 x 1023 of
something


1 mole of an element
contains 6.02 x 1023
atoms of that element
1 mole of water
contains 6.02 x 1023
molecules of water
 This number is known
as Avogadro’s number
 1 mole of an element (or compound) equals
that elements atomic mass (or formula mass)
in grams

This is called the Molar mass
 Formula mass of water =
 Molar mass of water =
 What is the molar mass of Nickel (III)
carbonate?
Mole Conversions
 Mass and Moles
 1 mol = molar mass (g)
 Particles and Moles

Particles = molecules, atoms, formula units,
ions, etc.
 1 mole = 6.02x1023 particles
 Moles and Volume of a Gas

This conversion only works at STP

0 degrees Celsius and 1 atm of pressure
 22.4 L / 1 mole
Multistep conversions
 What would be the mass in grams of 8.0 x
1024 molecules of sodium hydroxide?
 What volume would 15 g of oxygen gas
occupy at STP?
Percentage Composition
 The mass of each element in a compound
compared to the entire mass of the
compound and then multiplied by 100
 Can be calculated 2 ways:


1. From the chemical formula
2. From experimental analysis of a sample
1. From the chemical formula:
CaCl2
2. From experimental analysis of a sample:
Analysis of a 4 g sample of an unknown
compound reveals that it contains 2.3 g
sodium, 1.6 g oxygen, and 0.1 g hydrogen.
Find the percent composition.
 A sample of a compound has a mass of 5
grams and is determined to be 13% carbon,
2.2% hydrogen, and 84.8% fluorine. What
mass of each element is in this sample?
Empirical Formula
 The empirical formula gives the simplest
whole-number ratio of the atoms in a
compound.
1. Finding empirical formula from percent
composition.
-What is the empirical formula of a
compound that is 80% carbon and 20%
hydrogen?
2. Determine the empirical formula for a
compound containing 7.30 g Na, 5.08 g S,
and 7.62 g O.
Molecular Formula
 Molecular formulas show the actual number
of atoms of each element in a compound.
 Always
a whole number multiple of the
empirical formula
 Determined
by comparing the molar mass of
the unknown compound with the molar mass
of the empirical formula.
 Ribose is a sugar found in DNA and RNA. It
has a molar mass of 150 g/mol and a
chemical composition of 40% carbon, 6.67%
hydrogen, and 53.3% oxygen. Determine the
molecular formula.