Chapter 7 Chemical Quantities or How do you measure how much? You can measure mass, volume, or you can count pieces of a substance. We measure mass in

Chapter 7

Chemical Quantitiesor

How do you measure how much?

You can measure mass, volume,

or you can count pieces of a substance. We measure mass in grams. We measure volume in liters.

We count pieces in MOLES. No, not that kind of mole!!!

Moles Defined as the number of carbon atoms in

exactly 12 grams of carbon-12. 1 mole is 6.02 x 1023 particles. Treat it like a very large dozen. 6.02 x 1023 is called Avogadro's number. The units of Avogadro’s number can be

whatever particle you are counting.– Examples: atoms, molecules, ions, formula units,

etc…– In chemistry these are called Representative

Particles

What are Representative Particles?

The smallest pieces of a substance. For an element it is an atom.

– Unless it is diatomic

For a covalent compound it is a molecule. For an ionic compound it is a formula unit. If it has a charge, it is an ion.

How do We Use Moles? Moles are used as conversion factors. This means they are used to change units. Remember, when solving using conversion

factors there are 3 questions you want to ask yourself:– What unit do you want to get rid of?– Where does it go to cancel out?– What can you change it into?

Calculation question 1 How many molecules of CO2 are the in 4.56

moles of CO2 ?

Answer: 2.75 x 1024 molecules of CO2

Calculation question How many moles of water is 5.87 x 1022

molecules of water?

ANSWER: 0.0975 moles of water

Measuring Moles The atomic mass unit (amu) is one twelfth the

mass of a carbon 12 atom. Since the mole is the number of atoms in 12

grams of carbon-12, the decimal number on the periodic table is:– The mass of the average atom in amu.

This mass is equivalent to the mass of 1 mole of those atoms in grams.

Gram Atomic Mass

The mass of 1 mole of an element in grams. 12.01 grams of carbon has the same number

of pieces as 1.008 grams of hydrogen and 55.85 grams of iron.

We can write this as: 12.01 g C = 1 mole

We can count things by weighing them.

Examples

How much would 2.34 moles of carbon weigh?

28.1 grams of C How many moles of magnesium in 4.61 g of

Mg?

0.190 moles of Mg

What about compounds?

In 1 mole of H2O molecules there are two moles of H atoms and 1 mole of O atoms.

To find the mass of one mole of a compound: – Determine the moles of the elements they have.– Find out how much their mass is by using the

periodic table and then add them up.

Example What is the mass of one mole of CH4?

1 mole of C = 12.01 g 4 mole of H x 1.01 g = 4.04g

1 mole CH4 = 12.01 + 4.04 = 16.05g

The Gram Molecular mass of CH4 is 16.05g

Gram Molecular Mass or GMM The mass of one mole of a molecular

compound.

Gram Formula Mass - The mass of one mole of an ionic compound.

Calculated the same way as GMM. What is the GFM of Fe2O3?

2 moles of Fe x 55.85 g = 111.70 g 3 moles of O x 16.00 g = 48.00 g The GFM = 111.70 g + 48.00 g = 159.70g

Molar Mass

The generic term for the mass of one mole. The same as gram molecular mass, gram

formula mass, and gram atomic mass. This is the term we will be using in class.

I do NOT weigh that much! The balance must be lying!!!

Examples Calculate the molar mass of the following

and tell me what type it is (gmm, gfm, or gam).

Na2S

N2O4

C

Ca(NO3)2

C6H12O6

(NH4)3PO4

78.05 g/mol

92.02 g/mol

12.01 g/mol

164.10 g/mol

149.12 g/mol

180.18 g/mol

Using Molar Mass

Finding moles of compounds

Counting pieces by weighing

Molar Mass

The number of grams in 1 mole of atoms, formula units, or molecules.

We can make conversion factors from these. It will allow us to change grams of a

compound to moles of a compound. Or moles to grams.

For example

How many moles is 5.69 g of NaOH?

For example


NaOH g 69.5

For example


g

mole NaOH g 69.5

need to change grams to moles

For example


g

mole NaOH g 69.5

need to change grams to moles for NaOH

For example


g

mole NaOH g 69.5


1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g

For example


g

mole NaOH g 69.5


1mole Na = 22.99g 1 mol O = 16.00 g 1 mole of H = 1.01 g add these together to get the molar mass – use it to convert!

1 mole NaOH = 40.00 g

For example


g 40.00

mole 1 NaOH g 69.5




For example


NaOH mol 0.142 = 40.00

mole 1 NaOH g 69.5

g




Examples

How many moles is 4.56 g of CO2 ?

How many grams is 9.87 moles of H2O?

0.104 mol CO2

177.86 g H2O

Gases and the Mole

Where is my air tank??

I hope it’s filled to 22.4 L

GasesMany of the chemicals we deal with are gases.They are difficult to weigh, so we’ll measure

volume.We still need to know how many moles of gas

we have.Two things affect the volume of a gas:

– Temperature and pressure.So we have to compare at the SAME temp. and

pressure.

Standard Temperature and Pressure

Avogadro's Hypothesis - at the same temperature and pressure equal volumes of gas have the same number of particles.

0ºC and 1 atm pressureabbreviated STPAt STP, 1 mole of gas occupies 22.4 LCalled the molar volume.

ExamplesWhat is the volume of 4.59 mole of CO2 gas

at STP?

How many moles is 5.67 L of O2 at STP?

What is the volume of 8.8g of CH4 gas at

STP?

We have learned how to:

change moles to gramsmoles to atomsmoles to formula unitsmoles to moleculesmoles to liters

Volume

Molecules

Atoms

Representative Particles

MassPT

Moles

6.02 x 1023

22.4 L

Count

Percent Composition

Like all percents: Part x 100 %

whole Find the mass of each component, divide by the total mass.

Example

Calculate the percent composition of a compound that is 29.0 g of Ag combined with 4.30 g of S.

Examples Calculate the percent composition of C2H4.

Aluminum carbonate.

Percent to Mass Multiply % by the total mass to find the mass

of that component. How much aluminum in 450 g of aluminum

carbonate?

Empirical Formula

From percentage to formula

The Empirical FormulaThe lowest whole number ratio of elements in

a compound.The molecular formula is the actual ratio of

elements in a compound.The two can be the same. CH2 empirical formulaC2H4 molecular formulaC3H6 molecular formulaH2O both

Finding Empirical FormulasJust find the lowest whole number ratio.

C6H12O6 , CH4N2

It is not just the ratio of atoms, it is also the ratio of moles of atoms.

Calculating Empirical FormulasWe can get ratio from percent composition.Assume you have 100 g.The percentages become grams.Convert grams to moles. Find lowest whole number ratio by dividing

everything by the smallest moles.

ExampleCalculate the empirical formula of a compound

composed of 38.67 % C, 16.22 % H, and 45.11 %N.

Assume 100 g so: = 3.22 mole C

= 16.1 mole H

= 3.22 mole N

1 mol C38.67 g C

12.01 g C

1 mol H16.22 g H

1.01 g H

1 mol N45.11 g N

14.01 g N

Example Continued:The smallest number of moles is 3.22 mol so

divide your answers by 3.22 to get the mole ratio for the formula.

The ratio is:

So the formula is: C1H5N1

3.22 mol C = 1 C

3.22 mol16.1 mol H

= 5 H3.22 mol

3.22 mol N = 1 N

3.22 mol N

Additional Examples: A compound is 73.9 % Hg and 26.1 % Cl.

What is the empirical formula?

Caffeine is 49.48% C, 5.15% H, 28.87% N and 16.49% O. What is its empirical formula?

HgCl2

C4H5N2O

Empirical to molecularSince the empirical formula is the lowest ratio the

actual molecule would weigh the same or more.

This difference in weight would be by a whole number multiple.

To find this multiple: divide the actual molar mass by the the mass of one mole of the empirical formula.

You will get a whole number.

Multiply the subscripts of the empirical formula by this number.

Example A compound has an empirical formula of

ClCH2 and a molar mass of 98.96 g/mol. What is its molecular formula?

A compound has an empirical formula of CH2O and a molar mass of 180.0 g/mol. What is its molecular formula?

Cl2C2H4

C6H12O6

Percent Composition to Molecular Formula:Ibuprofen is 75.69 % C, 8.80 % H, 15.51 % O,

and has a molar mass of about 207 g/mol. What is its molecular formula?

C13H18O2

Example: Caffeine is 49.48% C, 5.15% H, 28.87% N

and 16.49% O. It has a molar mass of 194 g. What is its molecular formula? C 0.4948*194g = 95.99 g C = 8 mol C

H 0.0515*194g = 9.99 g H = 10 mol H

N 0.2887*194g = 56.01 g N = 4 mol N

O 0.1649*194g = 31.99 g O = 2 mol O

1 mol C95.99 g C

12.01 g C

1 mol H9.99 g H

1.01 g H

1 mol N56.01 g N

14.01 g N

1 mol O31.99 g O

16.00 g O

C8H10N4O2

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Chapter 7 Chemical Quantities or How do you measure how much? You can measure mass, volume, or you can count pieces of a substance. We measure mass in