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.

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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