the mole what is it? how do we use it? chapter 10

The MoleWhat is it? How do we use it?

Chapter 10

Calculating the mass of atoms:

Carbon-12 was used as the standard for relative mass.

A single atom of C-12 is assigned a mass of exactly 12 atomic mass units.

The symbol for atomic mass units is u or amu.

The mass of an atom expressed in atomic mass units is called the atomic mass of the atom.

Calculation of Formula Weight

Formula weight - sum of the atomic weight of each atom in the chemical formula.

Ex. Formula wt. of NaOH

FW = 1 (atomic weight of Na) + 1 (atomic weight of O) + 1 (atomic weight of H)= 22.989770 + 15.9994 + 1.00794 =

39.9971 amu

More PracticeEx. Formula wt. of Al(OH)3

1 amu Al + 3 amu O + 3 amu H =

26.981538+3(15.9994)+3(1.00794) =78.0036 amu

Measuring AmountsWe often measure the amounts of things by one of 3

methods:

By countBy massBy volume

For example, you can buy soda by the six-pack or the liter.

We can buy oranges by count or by mass (weight).

Ex. Oranges: 8 for $3.00 or $.79/lb

It’s easy to buy big items by count: apples, oranges, pens, etc. but sometimes it’s easier to buy them when they are grouped together:Ex. A dozen eggs

What about smaller items? How about grains of sand in a timer?

The Mole

Chemists use a number that is a specified number of particles to represent particles that are too small to count individually.

It’s the

The Mole1 mole is:

602,200,000,000,000,000,000,000

or 6.022 x 1023 representative particles

A representative particle means ITEMS: atoms, molecules, formula units

Molar Mass (aka Molecular Weight)

Molar mass (also known as molecular weight) - mass of one mole something.

Scientists made it easy by making molar mass in grams the same number as atomic mass in amu.

Units of molar mass: grams per mole. (g/mol)

What is the mass of one mole of aluminum?

26.981538 g

PracticeWhat is the mass of 2.3 moles of sulfur?

Known: S mm is 32.066 g/mol.

Unknown: mass (in grams)

Start with what we know:

2.3 mol S 32.066 g

1 mol

2.3 mol S 32.066 g S

1 mol

Set up the problem so that the unit you want is on the top

• Cross out the units that cancel and then multiply

2.3 mol S = 73.752 g

What if I know the grams and want to know the # of moles?

How many moles are in 412 grams of CaSO4?

Known: have 412 g. CaSO4

Molar mass of CaSO4 = 136.142 g/mol

412 g 1 mol CaSO4

136.146 g

= 3.03 mol CaSO4

PracticeHow many grams are in 2.4 moles of NaCl?

How many moles are in 77.4 g of BaCO3?

How many moles are in 3.45x104 g of copper (II) oxide?

What is the mass of 16 moles of Pt?

What is the mass of 22 moles of NO?

Based on chemical equations and the law of conservation of matter.

Must know the chemical equation for the reaction we are studying.

Stoichiometry

There are 4 types of reaction stoichiometry

problems:1. mole-mole problems:

moles of A → moles of B

2. mole-mass problems: moles of A → moles of B →mass of B

3. mass –mole problems: mass of A → moles of A → moles of B

4. mass-mass problems: mass of A → moles of A → moles of B →

mass of B

Mole ratioThe relationship between the number of moles

of reactants and/or products in a chemical equation.

N2 (g) + 3 H2 (g) → 2NH3 (g)

The mole ratio of hydrogen to nitrogen is 3 to 1

Now lets PracitceIn the Synthesis of water I start out with 42

grams of H2 and an excess of 02 how much water will I have at the conclusion of the reaction?

1. Write the equationH2 + O2 H2O

2. Balance the equation2H2 + O2 2H2O

3. Start with what you know

4. 42 g H2

Now its your Turn!

Butane (C4H10) undergoes combustion.

Balanced Equation:

__ C4 H10 + __ O2 __ CO2 + __ H2O

How many grams of CO2 are produced when 88 g of O2 are reacted with an excess of butane?

Volume / Mole Relationship (@ STP)

Percent CompositionTarget: Be able to calculate percent

composition & successfully complete the worksheet!

The percent by mass of an element in a compound is the number of grams of the element divided by the mass in grams of the compound, multiplied by 100%:

% mass of element = mass of element x 100%

mass of compound

Calculating Percent Composition from Mass Data

When a 13.60 g sample of a compound containing only magnesium and oxygen is completely decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound.

Think this through…how much magnesium do we have after decomposition…

When a 13.60 g sample of a compound containing only magnesium and oxygen is completely

decomposed, 5.40 g of oxygen is obtained. What is the percent composition of this compound.

A compound is formed when 9.03 g Mg combines completely with 3.48 g N. What

is the percent composition of this compound?

Percent Composition from Chemical Formula

% mass = mass of element in 1 mol compound x100%

molar mass of compound

Example: Propane (C3H8) is one of the compounds obtained from petroleum. Calculate the percent composition of propane.

Think…what is our mass of carbon & hydrogen in 1 mole of propane…how do we figure this out?

Propane (C3H8) is one of the compounds obtained from petroleum. Calculate the

percent composition of propane.

Calculate the percent nitrogen in these common fertilizers:

a. NH3

b. NH4NO3

Percent Composition as a Conversion Factor

Used to calculate the number of grams of any element in a specific mass of a compound

How:

Multiply the mass of the compound by a conversion factor (based on the percent composition of the element in the compound)

Example (using propane example): how much carbon & hydrogen are in

82.0 g of propane?

Think it through…what were the percentages of carbon & hydrogen in propane?

So…in a 100g sample of propane, how much carbon & hydrogen would there be?

Example (using propane example): how much carbon & hydrogen are in

82.0 g of propane?

Calculate mass of Carbon in 82 g of propane:

Calculate mass of Hydrogen in 82 g of propane:

Determining Chemical Formulas

Empirical Formula (aka Simplest formula):

consists of the symbols for the elements combined with the subscripts showing the smallest whole-number ratio of the atoms.

To find the empirical formula: Determine the composition by mass –

set the 100% to grams. Make each element’s percentage equal to grams.

Determine the composition in moles --divide each number of grams found in #1 above by the atomic mass of the element.

Find the smallest whole –number ratio of atoms --Divide by the smallest # of moles found in #2 above.

Example: Analysis shows a compound to contain 26.56% potassium, 35.41% chromium and 38.03%

oxygen. Find the simplest formula of this compound.

A compound is found to contain 63.52% iron and 36.48% sulfur. Find its simplest

formula

FeS

Finding Molecular FormulaTo find the molecular formula for a

compound we need to know the correct molar mass of the substance.

To determine the molecular formula we divide the molar mass by the simplest formula mass, this gives us a whole number, which we then multiply all of the atoms in the simplest formula by to get the molecular formula.

Example: Determine the molecular formula of a compound having a simplest formula of

CH and a formula mass of 78.110 amu

C6H6

A sample of a compound with a formula mass of 34.00 amu is found to consist of 0.44 g of hydrogen and 6.92

g. of oxygen. Find its molecular formula.

H2O2