Warm-Up
• Which best represents the electron
configuration for an atom of Mg+2?
• A)1s22s22p63s2
• B) 1s21p62s22p63s2
• C) 1s22s22p6
• D) 1s22s22p63s23p64s24d6
Practice
How many atoms are present in 23.46 grams
of Mn2(SO4)7?
How many kilograms are in 3.76 x 1024
particles of Fe(OH)3?
Practice
•
How many moles are present in 3.45 x 1023
atoms of Ar?
• How many atoms are present in 4.56 moles
of Fe?
• Convert 6.84 moles of Ca to particles.
Molar Volume
•Molar Volume is often used to describe a
gas
•Gases can change volume based on
temperature and pressure
•STP-standard temperature and pressure
Molar Volume
•Pressure: amount of force per unit area
•STP- is a certain set of conditions
•Standard Pressure is 101.3 kPa or 1atm
•Standard Temperature is 0C or 273 K
Molar Volume Continued
•The volume of 1 mole at STP is 22.4 L
•Therefore,
1 mole (at STP) = 22.4 L
Examples
•1) How many moles are in 45 L H2O at
STP?
•2) How many liters are in 12.3 moles of
H2SO4 at STP?
Classwork
•3) How many liters are in 1.5 x 10-10 moles of O2
at STP?
•4) How many particles are present in 23.4 L of a
gas at STP?
Example
•What volume is occupied by 90 g CO2 at STP?
•1) Determine conversion factors
•2) Use factor label
Gas Density and Molar Mass
•Recall,
•1 mole = 22.4 L
•1 mole = gfm
1 mole CO2 = 44 g CO2
1 mole CO2 (at STP) = 22.4 L CO2
90 g CO2
1 mole CO2 22. 4 L CO2
44 g CO2
1 mole CO2
= 45.8 L CO2
Review Questions
•
How many particles are in 54.0 g
NH3?
• How many grams are 3.4 x 1024
particles of SO3?
Practice
• Determine the number of particles in
34.5 g of H2SO4.
• How many grams are present in 2.34
25
x 10 particles of Br2?
Practice
•1) How many atoms are in 19.5 moles of Ca?
•2) How many grams are present in 0.54
moles of K2S?
•3). How many grams of H2SO4 are present in
25
9.524 x 10 atoms of H2SO4?
Empirical Formula
•The empirical formula {also
called the simplest formula}
gives the lowest whole number
ratio of atoms of the elements
in a compound
Empirical Formula
• In an empirical formula, the
number of atoms do not reduce
any further
• The subscript used must be
expressed as a whole number (no
decimal places)
Steps to Solve
First determine the number of moles of each
substance present
•Convert the grams of each element to moles
(using the gfm or molar mass)
• 1)
•2) Then divide each number of moles present
by the smallest number of moles
•3) (If Necessary) multiply the number of atoms
by a number that will yield whole numbers
Example #1
•What is the empirical formula of a
compound that is 60.0 % Magnesium and
40.0% oxygen?
Solution
•Assume that 100 g of the compound are present
•60.0% Magnesium= 60.0 g Magnesium
•40.0 % Oxygen = 40.0 g Oxygen
Solution Continued
60.0 g Magnesium
1 mole Magnesium
24.3 g Magnesium
= 2.46 moles Magnesium
Solution Continued
• Oxygen:
40.0 g Oxygen
1 mole Oxygen
16.0 g Oxygen
= 2.50 moles Oxygen
Solution Continued
•3) Now divide the number of moles for each
element by the smallest number of moles.
•2.46 moles Magnesium
•2.50 moles Oxygen
Solution Continued
•Magnesium has the smallest number of moles
{2.46 moles}
•So divide the number of moles of Magnesium
and Oxygen by 2.46
•Magnesium
= 2.46 moles / 2.46 moles
= 1 mole Magnesium
•Oxygen = 2.50 moles / 2.46 moles
= 1.02 moles Oxygen
•Mg1O1
•MgO is the empirical formula
Example #2
•What is the empirical formula of a compound
that is 25.9% nitrogen and 74.1% oxygen?
Solution
•Assume that you have 100 g of the
compound
•25.9% Nitrogen = 25.9 g Nitrogen
•74.1 % Oxygen = 74.1 g Oxygen
Solution Continued
•Convert the grams of each element to moles
(using the gfm)
25.9 g Nitrogen 1 mole Nitrogen
14 g Nitrogen
= 1.85 Moles Nitrogen
Solution Continued
• Oxygen
74.1 g Oxygen
1 mole Oxygen
16 g Oxygen
= 4.63 Moles Oxygen
Solution Continued
Now divide the number of moles for each
element by the smallest number of moles.
•1.85 moles Nitrogen
•4.63 moles Oxygen
• 3.
Solution Continued
•Nitrogen has the smallest number of moles
•So divide the number of moles of Nitrogen
and Oxygen by 1.85
Solution Continued
•Nitrogen
= 1.85 moles / 1.85 moles
= 1 mole Nitrogen
•Oxygen = 4.63 moles / 1.85 moles
= 2.50 moles Oxygen
•
N1O2.5
Solution Continued
•Atoms can never appear in fractions or
decimals
•Thus, you must multiply the number of atoms
by a number that will yield whole numbers
•In N1O2.5, multiply the atoms by “2”
•Thus N2O5is your final answer
Example # 3
•An 8.20 g piece of magnesium combines
completely with 5.40 g of oxygen to form a
compound. What is the empirical formula?
Additional Examples
•Find the Empirical Formula of the following:
•a. 71.65% Cl, 24.27% C, 4.07% H
•b. 43.64% P, 56.36% O
Practice
•Find the empirical formula of a compound that
contains:27.4% Na, 1.19% H, 14.3% C, and
57.1% O