Empirical Formulas from Percent Composition

One way of determining the identity of an unknown in a lab is by analyzing its mass to determine its empirical formula (lowest whole number ratio of each element in a compound).  There are several types of problems, but all of them use the same concepts to start.

Example 1:  An unknown substance is composed of 24.7% potassium, 34.7% manganese and 40.5% oxygen.  Determine the empirical formula for this compound.

Problem!  You can't compare percentage by mass to determine the ratio of ATOMS!

1. The first step is to convert the percentages to MOLES using the MOLAR MASS for each element.
2. Once you have all the substances in moles, you can compare them to find the mole ratio.  There are several ways of doing this.  The easiest is to divide by the smallest value.  This usually works, but remember that an empirical formula is written in the LOWEST WHOLE NUMBER ratio, so if you a left with a fraction, you must multiply the entire ratio by a factor that will convert the fractions into WHOLE NUMBERS.
   
3. Therefore the ratio of K:Mn:O is 1:1:4, so the empirical formula is KMnO4.

Percent Composition by Mass

Percent always allows us to compare a part of something to the whole.
In general

% = part x 100

total

For percent composition

% = total mass of particles requested x 100

molar mass

Example: Determine the % oxygen in sulfuric acid.

% O =       4 oxygen          x 100

(2 H + 1 S + 4 O)

%O =               4(16.0) ___  _  x 100 = 48.9% O

                                                         2(1.0) + 32.1 + 4(16.0)

Example: Determine the % sulfate in sulfuric acid.

%SO4 =               32.1 + 4 (16.0)___  _  x 100 = 97.9% O

                                                            2(1.0) + 32.1 + 4(16.0)

Mole Conversions

While we tend to measure amounts in grams, the only way to compare amounts of atoms, molecules or ions is by using moles.  Unit analysis allows us convert one set of units to another.

To convert grams to moles, or visa-versa, we use molar mass that has the units grams/1 mole.

To convert number of particles to moles, or visa-versa, we use Avogadro's number (6.02 x 10^23) that has the units particles/1 mole.

Example: How many chlorine atoms are in 75.0 grams of sodium chloride?

Hydrates

Hydrates are ionic salts that trap water molecules in their crystal lattice.  This added mass must be used when making calculations therefore the ratio between molecules of ionic salt and water is given in the name.

For instance, calcium sulfate hexahydrate states that for every molecule of calcium sulfate there are 6 water molecules surround it.  We represent a hydrate with a large dot then the number of water molecules.  This dot is NOT a multiplication sign, it is actually a ratio.

To determine the molar mass of hydrate, determine the mass of the salt then add the mass of however many water molecules are attached to it.

1 Ca + 1 S + 4 O + 6(2 H + 1 O)

40.1 + 32.1 + 4(16.0) + 6(18.0)

244.2 g/mole

The Mole

Mole is a term used in chemistry to represent the number 6.02 x 10^23.  Just as we use the word "dozen" to mean 12 objects, "mole" represents 6.02 x 10^23.

Amedeo Avogadro studied molecular theory in the early 19th century and built on the ideas of Dalton and Guy Lussac.  The number of particles in a mole was actually discovered later in the century and named in his honor.

Atoms and molecules are VERY small. Remember we measure their mass in atomic mass units, amu.  An amu is equal to 1/12th the mass of a carbon-12 atom or approximately the mass of a proton or neutron. The wonderful thing about the very odd number is it allows to work with measurable quantities.

One mole of atoms of any element is equal to its atomic mass (average mass number) in grams.  This is called molar mass.

The molar mass of a compound is simply the sum of the masses of each of its atoms.

water is H20
there are 2 Hydrogen and 1 Oxygen
therefore its molar mass is 2(1.0) + 1(16.0) or 18.0 grams/mole

This means that if you have 18.0 grams of water, you will also have 6.02 x 10^23 molecules of water.

Solubility Rules

Soluble means a substance will dissolve in water, whereas insoluble means it will not dissolve appreciably. If an insoluble product is created in an aqueous solution, it will precipitate out of solution as a solid.

Solubility Rules

Mainly soluble:

all nitrates & acetates

all halogens, except with silver, mercury and lead

all chlorates, except with silver, mercury and lead

all sulfates, except with calcium, strontium, barium, lead, mercury, and silver

all chromates, except with calcium, strontium, barium, lead, mercury, and silver

Mainly insoluble:

all sulfides, except with groups 1 & 2, and ammonium

all hydroxides, except with groups 1 & 2, and ammonium

all carbonates, except with group 1 and ammonium

all phosphates, except with group 1 and ammonium

Driving Forces

The driving force for a reaction is the reason the reaction moves forward.  It is the reason the reactants become the products.  To truly analyze the driving force, you must write the ionic reaction.  At this point in the semester, we will use the following reasons.

• a solid is produced from an aqueous solution
• a gas is produced
• water is produced
• a more active metal replaces a less reactive metal in a single replacement reaction
• a more reactive nonmetal replaces a less reactive nonmetal in a single replacement reaction
• neutral elements combine to form an ionic compound in a synthesis reaction

The last 3 reasons are types of redox reactions where electrons are transfered.  We will discuss redox reaction in the future.

Chemical Reactions: 4 Basic Types

Synthesis (also called Addition, Composition and Combination)

     Atoms and or molecules are combined to make ONE more complex molecule.

A  +  B --> AB

Fe  +  S -->  FeS

metal oxide + water --> metal hydroxide

nonmetal oxide + water --> oxyacid

Decomposition (also called Analysis)

     A complex molecule is broken into simpler molecules and or atoms

AB --> A + B

KClO3 --> KCl  +  O2

metal chlorate --> metal chloride + oxygen

metal carbonate --> metal oxide + carbon dioxide

Single Replacement

     A single element replaces one component of a compound.  A metal must always replace a metal and a nonmetal must replace a nonmetal.  A single replacement replacement is a type of redox reaction where electrons are transfered from one atom to another.

A  + BX --> AX + B

K + CsCl --> KCl + Cs

X + AY --> AX  + Y

Cl2  +  MgBr2 --> MgCl2  +  Br2

Double Replacement (also called Ionic and Precipitation Reactions)

     Ionic compounds switch "partners."  Remember that a neutral compound must have a cation combined with an anion.

AX  +  BY --> AY  +  BX

CuCl  +  NaS --> CuS  +  NaCl

Naming Acids

Naming Acids

 The most fundamental definition of an acid is an anion combined with H+ to form a neutral compound.

All acids begin with a H⁺  and are combined with one of the three types of anions.

            -ide             to            hydro----ic acid                        H2S          hydrosulfuric acid

            -ate             to                        -ic acid                        H2SO4      sulfuric acid

            -ite              to                        -ous acid                     H2SO3      sulfurous acid

                        I -ate something and it made me s-ic.

                                        and you m-ite give it to -ous.

Ionic Compounds

Ionic Compounds

Ionic compounds are formed between oppositely charged particles called ions.  Ions are formed when electrons are lost, called cations, or when electrons are gained, called anions.

A molecule must be neutral.  The total amount of positive charges must cancel out the total amount negative charges to create an overall charge equal to zero.

Covalent Compounds

Covalent Compounds

Covalent bonds are formed by 2 or more atoms sharing electrons.  Typically this is between 2 non-metals since both atoms want to gain electrons to acquire a full outer shell.

To name covalent compounds, use the following prefixes to represent the subscripts and change the suffix of the more electronegative (always on the right) element to -ide.

1  mono

2  di

3  tri

4  tetra

5  pent

6  hex

7  hept

8  oct

9  non

10 dec