Chemical Quantities
Chapter 7
Atomic Mass (Weight)
 The mass of an element expressed in
grams.
 Examples:
Carbon – 12.0 g
Sulfur – 32.1 g
Iron – 55.8 g
Mass of a Mole of a Compound
 1st you must know the formula of the
compound
 The formula tells you how much of each
element that is present.
 Example:
SO3 – 1 Sulfur atom & 3 Oxygen atoms
 After you know the formula then you
calculate the mass of the compound.
Molar mass
 Mass (grams) of one mole of the
substance.
 Once you know the formula of the
compound then you can calculate the
molar mass of that compound.
Molar Mass Cont.

1.
2.
3.
4.
Find the molecular mass of each
compound.
Li2S
FeCl3
Ca(OH)2
N2O5
Percent Composition
Grams of element
% mass of element = Grams of Compound X 100
Or
% Comp. = Part
X 100
Whole
Practice Problem
 Ethane (C6H6) ---- 222.06 g of C6H6
mole of C = 216.00 g
mole of H = 6.06 g
Find the % Composition of Carbon &
Hydrogen in Ethane (C6H6).
Significant Figures &
Scientific Notation
 All the digits that can be known precisely
in a measurement, plus a last estimated
digit.
 Expression of numbers in the form
n x 10 n where n is equal to or greater
than one and less than 10
H2O – H -- 2 x 1.01 = 2.02
O -- 1 x 16 = 16
Total: 18.02 g
 How much Hydrogen is in Water?
 What is Hydrogen’s % Composition in Water?
 What would happen to the % of hydrogen with 2
molecules of Water?
What is a mole?
 Amt. of a substance that contains
6.02 X1023 representative particles of
that substance.
 A unit of measurement
Avogadro’s number
 6.02 X 1023 – an
experimentally
determined number
• Standard Temperature and
Pressure ( STP) – is the condition
under which a gas is measured.
a. Standard temperature is 0oC, or
273 K
b. Standard pressure is 1atm,
101.3 kPa, 760 torr, or 760
mmHg
• 1 mol of any gas at STP = 22.4 L
Representative Particle
 Refers to the species present in a
substance: usually atoms, molecules, or
formula units. (ions)
 Example:
 Fe is composed of Iron atoms.
 K is composed of Potassium atoms.
Example: How many moles of
Mg is 1.25 x 1023 atoms of Mg?
Know:
 # of atoms = 1.25x1023 atoms Mg
 1 mole Mg = 6.02x1023 atoms Mg
Unknown:
 Moles = ? Moles Mg
 Desired conversion: Atoms ------ Moles
 Calculate the answer.
Practice Problems
 How many moles are 2.80 x 1024 atoms
of silicon?
 How many molecules is 0.360 moles of
water?
Amt. in Compounds
 Carbon dioxide has 3 atoms.
 1-carbon 2-oxygen
 Thus one mole of CO2 contains three
times Avogadro’s # of atoms.
Example: How many atoms are in
2.12 mole of propane (C3H8)?
Know:
 # of moles = 2.12 mole C3H8
 1 mole C3H8 = 6.02 x 1023 molecules C3H8
 1 molecule C3H8 = 11 atoms
( 3 carbon atoms & 8 hydrogen atoms)
Unknown:
 # of atoms = ? Atoms
Desired conversion: moles – molecules—atoms
Practice Problems
 How many atoms are there in 1.14 mole
SO3?
 How many moles are there in 4.65 x 10 24
molecules of NO2?
Mole -- gram
 How many grams are in 3.32 mole K?
 How many grams are in 4.52 x 10-3 moles
of C20H42?
Gram -- Mole
 How many moles are in 3.70 x 10-1 grams
of Boron?
 How many moles are in 27.4 g TiO2?