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Chemical Quantities
Chapter Overview
mass
gramatomic mass
gramformula mass
gram molecular mass
molarvolume
mole
count volume length
measuring matter
Chemical Quantities
Measuring Matter• Mass
• ________
• Counting• ________
• Volume• _____________
• Length• ________
Counting but not really counting
If you know the mass of ______ item you can find out how many items there would be in a greater quantity.
How many apples are there in 11.0 kg?
Step 1
11.0 kg X conversion =
factor
Given1 dozen apples = 2kg (mass)
How many apples are there in 11.0 kg?
Step 1
11.0 kg X _ dozen apples = _ kg
Given1 dozen apples = 2kg (mass)
____ dozen apples
Answer
• ____ apples
This method is counting but not really counting because it is an approximation of the answer.
Step 2 5.5 dozen apples * 12 apples = 1 dozen apples
Stoichiometry
The study of relationships that can be derived from chemical formulas and equations.
QuickTime™ and aGIF decompressorare needed to see this picture.
Formula Mass
formula mass• if given in ________ or ______
molecular mass• if substance is molecular
molar mass• if given in ______
Sum of the ________ ________
also calledgram atomic massgram formula mass
THE MOLE• Number of particles
– like the problem with the apples, particles are also represented in this way except they are called moles instead of dozens.
• Avogadro’s number
–________– this number is the unit particles per mole
• unit referring to molecules, atoms, and formula units
Molar Volume• ___ _____by one mole of particles
–If it is a solid or liquid find the mass of one mole and divide by _______
–If it is a gas:• One mole occupies _______ at STP
»STP = 0 °C and 101.3 kPa
Problems involving moles
• Example
–What is the molar mass of H2O?
Calculating molar mass_______ the masses of each of the separate elements in the formula
EXAMPLE FIND THE MOLAR MASS OF WATER
• Step One
–first find the molar mass of hydrogen.
–Each hydrogen atom contains 1.0 gram per mole. Multiply one by two because there are two hydrogen atoms.
• Answer to step one:2.0 grams
–The mass of one oxygen is 15.99.But you can round up to 16.
–There is only one oxygen molecule so multiply 16 by one.
–Answer to step two:
•16 grams
Step Two – find the mass of the oxygen molecules.
• Step Three– add the masses of the oxygen
and the hydrogen together.
–16 g + 2g = _______ g
–Therefore the molar mass of H2O is
• _______ grams per mole.
“Moles” is central
MolesMassX
÷
Molar mass
Convert mass to moles
• To convert mass to moles you need to ______ the mass by the molar mass.
ExampleHow many moles are in 4.0 grams of H2O?
How many moles are in 4.0 grams of H2O?
• Step one–find the molar mass of the formula
–as in the previous example take 1.0 times 2 for the hydrogen and 16.0 for the oxygen.
–Add them together to get 18.0 grams
Step 2 •take the 4 grams of H2O and _______ them by the molar mass of water to convert mass into moles.
• The math you would use for step 2 is as follows:
• 4.0g H2O X 1 mole H2O = . ___ mole 18 g H2O
This answer can also be written as _________ mole
Converting moles to mass
• Converting moles to mass is similar to converting mass to moles because you are taking the equation backward
• To convert, multiply mass by molar mass.
Example: How many grams are in 4 moles of H2O?
• Step One
–find the molar mass of the formula.
–From previous examples we know that the molar mass of H2O is 18 grams.
• Step Two
–once you find the 18 grams for the molar mass of water, take the 4 moles of water and _______ by the molar mass.
• The math equation for step two is shown below:
• 4 moles H2O X 18 grams H2O =
1mole H2O
• The answer is ____ grams.
• Notice in the equation above the moles cancel out to leave the grams for the answer.
Convert Molecules To Moles
• To convert _______ to moles divide the number of molecules by Avogadro’s number (_______).
• One mole of any substance has the same number of molecules per mole
• Example–How many moles are in
1.806 X 1024 molecules of water?
Take the number of molecules and divide by Avogadro’s number
• The math equations would look as follows:• 1.806X1024 atoms X 1 mole water =
_______
The answer :_______ moles of water.
Once again notice that the atoms cancel each other out.
“Moles” is central
MolesMass
Molecules
XX
÷
÷ Avogadro’s numberMolar mass
Converting Moles to Molecules
• To convert moles to molecules _______ the moles by Avogadro’s number (6.02X1023)
Converting Moles to Molecules
• Example–How many molecules are in 173
moles of H2O?
• Step One–Take the number of moles and
_______ by Avogadro’s number
• The math equation would look as follows:
•173mole X 6.02X1023molecules =
1 mole
The answer is:____________ molecules
Gas Volume at STP to Moles
• STP = 0°C and 101.3 kPa
• To convert gas volume to moles–_______ the number of Liters by
22.4L/mole (Molar volume of any gas at STP)
EXAMPLEHOW MANY MOLES ARE IN 34.6 L
OF H2O(g)?
• 34.6 L ÷ 22.4 L/mole
= 1.54 mole
Volume ÷ molar volume = moles
At STP
“Moles” is central
Moles
Volume, gas
Mass
Molecules
X
XX
÷÷
÷
Molar volume
Avogadro’s numberMolar mass
Moles to Gas Volume
• To convert moles to gas volume you have to take the number of moles and _______ it by 22.4 L/mole if conditions are at STP.
EXAMPLE
• Take 73.6 moles X 22.4Liters/mole
• _______ Liters at STP
How many Liters are there in 73.6 moles of H2O(g) ?
Combination Calculations Example• How many molecules are in 29.43 Liters of
oxygen gas at STP?
•29.43 L X 1 mole = ______mole 22.4 L•1.31 mole X 6.02 X 10 23 molecules = 1 mole
•29.43 L ÷ ______Liters/mole =
•____________ molecules
Mass of one atom
• Divide the molar mass by Avogadro’s number
Mass of one atom
• What is the mass of one oxygen atom?
16.0 grams ÷ 6.02 X 10 23 atoms mole 1 mole
16.0 grams X 1 mole__________ =mole 6.02 X 10 23 atom
__________ g/atom
Gas Density
• The density of a gas is usually measured in grams per LITER
• It can be used to calculate the molar mass of a gas.
Molar mass from gas density
• Density = mass / volume
• If the mass you’re looking for is Molar Mass then use the Molar Volume
• The molar volume of any gas at STP is 22.4 Liters / mole
Molar mass from gas density• What is the molar mass of CO2 if its density at
STP is 1.96 g / L ?• D = Molar mass
Molar volume• 1.96 g / L = Molar mass
22.4 L / mole• Molar mass = 1.96 g X 22.4 L
L mole
• Molar mass = _____g / mole
Percent Composition by Mass• Percent composition is the relative
amounts of the elements in the compound.
• To calculate the percent composition,
take the ___of the required ____ and
divide by the _______ of the compound.
Percent Composition Example
• Calculate the percent composition of propane, C3H8.
• First get the total mass of the compound
• 3 moles C X 12 g/mole = 36 grams C
• 8 moles H X 1 g/mole = 8 grams H
• Total mass of propane = 44 grams
Percent Composition C3H8
• %C = 36.0 grams C X 100 44.0 grams propane
= _______ %
• %H = 8.0 grams H X 100 44.0 grams propane
= _______ %
Determining Empirical Formula
• The empirical formula shows the lowest whole number of _______ in a compound.– For example the empirical formula of
benzene, C6H6 is CH.
A 1 : 1 ratio
Determining Empirical Formula
• To calculate the empirical formula first get the
number of ____of each element
• Then get the lowest ____ ____ ____ of those moles.
– These numbers become the ________• If data is given as percent assume a 100 gram
sample and just change the percent to grams.
Example Empirical Formula
• What is the empirical formula of a substance that is 80.0% C and 20.0% H by mass?–First change % to grams by
assuming a 100 g sample•80.0 g C and 20.0 g H
Empirical Formula Example• Convert grams to moles (÷molar mass)
• 80.0 grams ÷ 12.0 grams/mole C• = 6.67 moles C
• 20.0 grams ÷ 1.01 grams/mole H• = 19.8 moles H
• The ____ of moles of H to C is calculated 19.8 moles / 6.67 moles = 2.97 or 3 : 1
Empirical Formula
• Therefore the Empirical formula of the compound is CH3
Molecular Formula
• The molecular formula shows the
____ ____ of ____ atom in the molecule.
– For example: C6H6 shows ____
atoms of carbon and ____ atoms of hydrogen in the benzene molecule
Molecular Formula• To calculate the molecular formula from
the empirical formula– First find the molar mass of the empirical
formula
– Next find the molar mass of the molecule
– Then ____ the molar mass of the molecule by the ____ ____ of the empirical formula to find how many times ____ the molecule is than the empirical formula.
If the molar mass of ethane is 30.0 grams per mole and the empirical formula as determined in a previous example is CH3,
what is the molecular formula of ethane?• First: The molar mass of CH3 is
12.0 + 3(1.01) = 15.0 g/mole• Second: The molar mass of ethane is
30.0 g/mole• Third: The ratio of molecule : empirical
is 30.0 g/mole ÷ 15.0 g/mole = ____
So the molecular formula is ____ times ____ than the empirical formula.
2 X CH3 = C2 H6
CREDITS
• Produced by
–Jay Seymour
–Patricia Brown
–1999