Chapter 7Chapter 7Chemical QuantitiesChemical Quantities

Fall 2008

The Mole: A Measurement of The Mole: A Measurement of Matter-What Is a Mole?Matter-What Is a Mole? How do you measure matter?

You count things You weigh things or determine

mass You get the volume

Can you count the grains of sand? Would you want to?

How do we determine the amount of something big or small without having to count it?

The Mole: A Measurement of The Mole: A Measurement of Matter-What Is a Mole?Matter-What Is a Mole? We use problem solving steps to

figure out the amount of very small or very big forms of matter. That method is called……

Dimensional AnalysisDimensional Analysis Here is an example:

How many eggs can Bug Bunny buy for cupcakes for Elmer Fudd’s birthday if he has $10 to buy eggs with at $0.80 cent for a dozen? First start with what you know

12 eggs = 1 dozen, 6 eggs = ½ dozen Eggs cost ¢80 for 1 dozen Bugs has $10

Dimensional AnalysisDimensional Analysis

Next we set up Conversion factors 12 eggs/ 1 doz. Or 1 doz./12 eggs $0.80/ 1doz. Or 1 doz./$0.80

Now we can set up the problem $10 1 doz. 12 eggs = 150 eggs

$ 0.80 1 doz.

What is a Mole?What is a Mole? But now back to the mole…

In chemistry, it is not the furry little creature…

It is the measurement of the amount of particles in a substance Particles can be atoms, ions, formula units or

molecules

What is a Mole?What is a Mole?

So, how much is a mole…..6.0223 x 1023 particles

Particles are extremely small and that number is extremely big.

This is due to Amadeo Avagadro’s research leading to the discovery, not the discovery of the number itself.

What is a Mole?What is a Mole?

It has also been discovered that 1 mole of all gases at a certain set of conditions called STP (standard temperature and pressure) have the same volume—22.4L!

So, 1 mole = 22.4L (gas @ STP)

Mass and Moles

How do we usually measure out chemicals in a lab? NOT by counting the number of particles We usually use mass to measure out

chemicals. The problem with mass is that not all atoms or compounds have the same mass.

Avogadro’s number was established by using Carbon-12 as a standard. The number of particles in 12 grams of carbon 12 was set as 1 mole.

Mass and Moles Conveniently, this is similar to how the

atomic mass unit was established, so the atomic mass on the periodic table is also an element’s molar mass (the mass of one mole of an element, measured in g/mol).

Since compounds are just atoms bonded together, you can find the molar mass of a compound by adding together the molar mass of all the elements in the compound.

Mass and Moles

We have now discussed moles in three ways—as particles (1 mol = 6.02x1023 particles), as a mass ( molar mass = 1 mol) and as a volume (22.4 L = 1 mol)

To get from particles to grams, grams to liters or particles to liters of a substance, two steps are required, with moles always in between!

The Mole MapThe Mole Map

Mole

Mass

Gas Volume @ STP

# Particles

x

x

x

22.4L6.02 x 10

23

Mol

ar

Mas

s

Mole Road MapMole Road Map

Here is a graphic of the procedure steps:

Pick the box of the data you are given in the problem and follow the steps toward the box containing what you are asked for in the problem.

% Composition% Composition The percent composition of an

element is the relative amount of that element in a compound.

You find the percent composition of an element by dividing the element’s mass by the mass of the entire compound.

mass of elementmass of mass of compoundcompound

x 100x 100

For example:For example:

What is the % composition of carbon dioxide?

Mass of oxygen = 2(16.0)=32.0 gMass of carbon = 12.0 gMass of carbon dioxide = 44.0 g 12.0 g C 44.0 g CO2

32.0 g O44.0 g CO2

x100 =x100 = 27.3% Carbon27.3% Carbon

x100 =x100 = 72.7% Oxygen72.7% Oxygen

% Water in a Hydrate% Water in a Hydrate

A A hydrate hydrate is an ionic compound is an ionic compound with water as part of its crystalline with water as part of its crystalline structure.structure.

To find the percent of water in a To find the percent of water in a hydrate, use the same formula, hydrate, use the same formula, except: except:

% Water =% Water = x 100 x 100Mass water Mass water Mass Mass

compoundcompound

Empirical FormulaEmpirical Formula The empirical formula gives you the lowest,

whole number ratio of elements in the compound.

The empirical formula may or may not be the same as the molecular formula.

For carbon dioxide, the molecular formula is CO2, and the empirical formula is CO2. One carbon and two oxygens are the lowest ratio of atoms.

The molecular formula for dinitrogen tetrahydride is N2H4, but the empirical formula is NH2.

What is the empirical formula for:What is the empirical formula for:

C6H12O6

N2H2

CH4

To Find the Empirical Formula To Find the Empirical Formula from % Composition:from % Composition:

If given the percentages, assume there are 100.0 grams of the compound.

Convert the grams of each element to moles. Divide by the smaller amount of moles, then

manipulate the ratio so that all numbers are whole.

What is the empirical formula of a compound that is 27.3% carbon and 72.7% oxygen?

What is the empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen?

To Find the Empirical Formula To Find the Empirical Formula from % Composition:from % Composition:

If given the percentages, assume 100% = 100.0 grams .

Convert the grams of each element to moles.

Divide by the smaller amount of moles, then manipulate the ratio so that all numbers are whole.

From there, find it’s molecular From there, find it’s molecular formula:formula:

If given the molar mass (how many g/mol of the compound) then you can calculate the molecular formula from the empirical formula.

Take the compound’s empirical formula mass and compare to the molecular mass. The molecular mass will be a multiple of the empirical formula’s mass.