Chemical Quantities

Standards 3b. Students know the quantity of one mole is

set by defining one mole of carbon-12 atoms to have a mass of exactly 12 grams.

3c. Students know one mole equals 6.02 x 1023 particles (atoms or molecules).

3d. Students know how to determine the molar mass of a molecule from its chemical formula and a table of atomic masses and how to convert the mass of a molecular substance to moles, number of particles, or volume of gas at standard temperature and pressure.

Objectives Know the meaning of the mole Know how to convert between moles,

grams, and number or particles Know how to calculate the molar mass

of a compound Know how to calculate percent

composition Know how to determine empirical,

molecular and hydrate formulas

Identity of some basic “particles” in chemistry

Element

Atom

Molecule…diatomic elements

Compounds Covalent vs Ionic

Covalent Compounds

Are composed of molecules

Compounds

Covalent vs Ionic

Ionic Compounds Are composed of positive and negative ions;

the representative particle is called a formula unit

Formula unit: the simplest ratio of ions represented in an ionic compound

Mole Defn: a counting reference. A quantity chosen by chemists to

represent the amount of a substance.

It is the number of representative particles in exactly 12 grams of

pure carbon-12

The amount of a substance which contains “6.022 x 1023 particles”

Particles can be: Atoms, molecules, formula units, ions, etc.

6.022 x 1023 is known as Avogadro’s Number

1 mole = the atomic mass of that element in grams...from the PT

Question

Where do you find the atomic mass for an element?

12 grams of carbon contains the same # of atoms as

19 grams of fluorine or 78.96 g of Se because they

each contain 1 mole of particles

1 mole of He = ________g He

58.69 g Ni = ________mole Ni

Atomic Mass

Atomic Mass: the weighted average mass of

the isotopes of an element…the value on the PT

What is the atomic mass of hydrogen?

1.00794 g

What is the atomic mass of Oxygen?

15.999 g

Avogadro's Number Avogadro's Number is 6.022x1023

which is the number of representative particles in a mole

6.022x1023 “particles” = 1 mole

Mole Conversions

Determine the “given” and the “get”

Mini road map

Set up equation with conversion factor(s)

Cancel the units and calculate.

Does your answer make sense?

MOLES ↔ GRAMS MOLES ↔ ATOMS

Example

2.00 moles Boron = how many grams

Boron?

Example

How many moles of calcium are there in

23.20 grams of calcium?

Example

How many atoms are there in 5.25

moles of nitrogen gas?

Put the Moles in the Middle!

GRAMS ↔ MOLES ↔ ATOMS

“MOLES IN THE MIDDLE,”

you gotta go through the MOLE!

Generic Mole Conversion Set-Up

Example

How many atoms are there in

27.35 g of aluminum?

Example

How many grams are there in 6.7 x

1024 molecules of HF?

Molar Mass

Defn: the sum of the atomic masses of

all the elements in a compound/element

Calculating Molar Mass

1) Determine the atomic mass of each element in the compound The atomic mass is found on the periodic

table

2) Add all the atomic masses together, this is your molar mass

EX. What is the molar mass of water?H2O: 2 H = 2 ( 1.00794 g) 1 O = 15.9994 g/mol

2 ( 1.00794 g) + 15.9994 g= 18.0153g/mol

Question

What is the molar mass of calcium

sulfate?

Percent Composition The percent by mass of each element in a

compound To determine the percent composition of a compound:

1) Determine the molar mass of the compound2) Determine the molar mass of the element3) Determine the percent of each element in the

compound, using the % composition formula4) Check to make sure percentages add up to 100%

% El = (# atoms of El) (atomic mass El) x 100

molar mass of compound

Question

What is the percent composition of H2O? % El = (# atoms of El) (atomic mass El) x 100

molar mass of compound

% H = (2) ( 1.00794 g) x 100 = 11.2% H 18.0153 g

% O =?

100-11.2=88.8%

Examples Calculate the percent composition of the following

compounds:

C2H6 CO2 NaHSO4

CO2

NaHSO4

Empirical Formulas

Defn: the formula with the smallest whole number mole ratio of

elements in a compound

If given percent composition, then:

1) Assume a 100 gram sample

2) Find the number of moles of each element

3) Divide each mole number by the smallest mole number

4) Write the formula

If you come up with a decimal number, multiply the decimal number by

a whole number to get a whole number

Then multiply all other ratios by that same whole number

Example

Given: 94.11% O and 5.89% H

Assume 100 g sample→

Example

Given: 52.9% C and 47.1% OAssume a 100 g sample→

Empirical Formulas

If given the mass of a compound and the mass of the

individual elements, then:

1) Determine how many grams of each element is present

2) Find the number of moles of each element

3) Divide each mole number by the smallest mole number

4) Write the formula

If you come up with a decimal number, multiply the decimal

number by a whole number to get a whole number

Then multiply all other ratios by that same whole number

Question Analysis of 20.0 g of a compound containing only

calcium and bromine indicates that 4.00 g of calcium are present. What is the empirical formula of the compound formed?

Molecular Formulas Defn: The actual number of atoms of each element in one molecule or

formula unit of the substance

You calculate a molecular formula from an empirical formula

To find the molecular formula:

1) find molar mass of empirical formula

2) Divide molar mass (given) by empirical formula mass to get an “integer”Molar mass of compound = N (an integer) empirical formula mass

3) Multiply empirical formula by the “integer”

(Molecular Formula) = (Empirical Formula)N…N distributes to all the subscripts

Example The molar mass of a compound is 42 g/mol.

Its empirical formula is CH2. What is the molecular formula for this compound?

Example What is the molecular formula for a compound with an

empirical formula of OH and a molar mass of 34 g/mol?

Hydrate Formulas

Defn:a solid which crystallizes with water molecules bonded to the compound in the crystalline lattice.

Name of the hydrous compound:

BaCl2 •2H2O → BaCl2(s) + 2 H2O (g) BaCl2 •x H2O → BaCl2(s) + x H2O (g)

To Name a hydrous compound:

Name of the ionic compound + prefix hydrate…(the prefix corresponds to the coefficient in front of the

water)

Prefixes: 1-mono 4-tetra 7-hepta2- di 5-penta 8-octa3-tri 6-hexa 9-nona

10-deca

Finding a Hydrate Formula

1) Find the number of moles for each

compound2) Divide each mol number by the smallest mol

number

3) Write the formula

4) Name the compound

Example A hydrate of CuSO4 is heated to drive off the

water of crystallization. When 10.0 g of the hydrate is heated, 6.39 grams of solid residue remain. Find the hydrate formula.

The End