Chapter 7Chapter 7Chemical QuantitiesChemical Quantities

Spring 2012

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.

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?

What is a Mole?What is a 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

How much is a mole?

It is an amount, defined as the number of carbon atoms in exactly 12 grams of carbon-12.

1 mole = 6.02 x 1023 of the representative particles.

Treat it like a very large dozen 6.02 x 1023 is called: Avogadro’s number.

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 are Representative Particles? The smallest pieces of a

substance:1) For a molecular compound: it is the

molecule.2) For an ionic compound: it is the

formula unit (made of ions).3) For an element: it is the atom.

Remember the 7 diatomic elements? (made of molecules)

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.

Molar Mass Molar mass is the generic term

for the mass of one mole of any substance (expressed in grams/mol)

The same as: 1) Gram Molecular Mass

(for molecules) 2) Gram Formula Mass (ionic compounds) 3) Gram Atomic Mass (for elements)

molar mass is just a much broader term than these other specific masses

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 another 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 element

mass 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

Example Calculate the percent composition of a compound that is made of of Ag with S.

Ag2S = 248 g

216 g Ag

248 g totalX 100 = 87.1 % Ag

32 g S

248 g totalX 100 = 12.9 % S

Total = 100 %

% Water in a Hydrate% Water in a Hydrate

A A hydratehydrate is is an ionic compound 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

C6H12O2

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. (100% = 100 g)

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.

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

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.

Formulas (continued)

Formulas for molecular compounds MIGHT be empirical (lowest whole number ratio).Molecular:

H2O

C6H12O6 C12H22O11

Empirical:

H2O

CH2O C12H22O11

(Correct formula)

(Lowest whole number ratio)

Example:Example:

If the molecular mass of the first example (N2O5) problem is 216 g/mol, then what is the molecular formula for the compound?